OIL AND GAS FIELDS OF LOUISIANA:
the NEBO-HEMPHILL FIELD,
Paul N. Lawless, M.S., & George F. Hart, Ph.D.,
Basin Research Institute,
Louisiana Geological Survey,
School of Geoscience,
June 1st., 1990.
The LaSalle Arch is a southerly trending anticline in north-central Louisiana. The northern end of the Arch separates the Louisiana and Mississippi interior salt basins. The structural trend of the arch is supported by relict paleo-highs. The paleo-high beneath the Nebo-Hemphill Field, as seen on reflection seismic data, is the nose of an Ouachitan thrust fault which was partially rifted during the opening of the Gulf of Mexico. The western limb of the Arch formed by differential subsidence expanding the stratigraphic section toward the southwest. The eastern limb of the Arch formed by regional tilting to the east after deposition of the Claibornian Sparta Formation. Late Cretaceous uplift of the LaSalle Arch is seen as a truncational unconformity within the Tayloran Demopolis Formation.
The Wilcox and Midway groups of central Louisiana have been subdivided into three genetic sequences which were bounded above and below by regionally correlatable flooding surfaces. They are T1 (the Midway), T2 (the Holly Springs), and T3 (the Carrizo). This sequence stratigraphic framework correlates with other studies in the type section in Alabama.
The highstand systems tract of T1 and T3 were deposited during a relative sea level fall resulting in similar homogenous wave dominated deltaic morphologies and electrofacies. Channel facies within T1 and T3 lie over the rift graben and thin over the thrust fault nose. This indicates subsidence played a key role in the location of depositional environments within T1 and T3, but did not prevent progradation to the southwest. The structural trend of the LaSalle Arch was unmodified during T1 and T3. The highstand systems tract of T2 was deposited during a relative sea level rise resulting in heterogenous fluvial dominated deltaic morphologies and electrofacies capped by a meandering channel system. The structural trend of the LaSalle Arch was highly modified during the T2 highstand systems tract. Channel facies are not controlled by the rift graben or thrust fault nose indicating subsidence did not play an important role in the location of depositional environments within T2.
Major hydrocarbon reserves are found over the nose of the thrust fault and to the northwest along the structural trend of the LaSalle Arch within the upper part of genetic sequence T2. Minor hydrocarbon reserves are found off of the structural trend of the Arch within the upper part of genetic sequence T3. No hydrocarbons have been found along the structural trend of the LaSalle Arch, were the few well penetrations are located, of genetic sequence T1. Added deeper pool potential may exist in sandy facies of T1 on the downthrown side of normal faults, which lie further to the south, north, and east of Nebo-Hemphill Field, where potential reservoirs would be juxtaposed against Midway shale.
Geochemical analysis of liquids and rock samples has shown hydrocarbons found within the Nebo-Hemphill Field to be similar to liquid hydrocarbons found in the expanded Tertiary reservoirs in southern Louisiana and dissimilar to pyrolytic products derived from local organic matter. Rock-Eval pyrolysis has shown organic matter from the Wilcox Group of the study area to be type III and immature.
Nebo-Hemphill Field is located in T7N,R3E LaSalle Parish, Louisiana over the southern end of the LaSalle Arch (figure 1). Production in the field is predominantly found in the middle part of the Wilcox Group and belongs to the updip Wilcox Trend. The trapping mechanism for the main reservoirs within the Nebo-Hemphill field is partially structural, the LaSalle Arch, and partially stratigraphic, updip stratigraphic pinchouts. The H.L. Hunt, Goodpine A #1 Sec10,T7N,R3E (LA# 24685 and API# 170590291900) was the discovery well for the field which spud on August 15, 1940, drilled to a TD of 3694 ft, and perforated from 3480 ft to 3490 ft. Total production to date (October 1989) and 1987 annual production for the field are as follows:
Crude Oil Production (Total): 72,561,334 bbls
Crude Oil Production (1987): 900,881 bbls
Condensate Production (Total): 4,295 bbls
Condensate Production (1987): NA
Natural Gas Production (Total): 41,372,296 ft3
Natural Gas Production (1987): 406,000 ft3
The natural gas production is predominantly casing head gas. Gas reservoirs do exist, but are seldom produced due to the economics. Average initial production values were obtained for each of the productive intervals within the middle Wilcox.
INTERVAL INITIAL OIL PRODUCTION INITIAL WATER PRODUCTION
A-zone 94.6 bbls/day 71.4 bbls/day
B-zone 137.3 83.2
C-zone 102.8 83.9
D-zone 123.7 143.6
E-zone 98.4 91.2
F-zone 138.9 84.4
G-zone 115.2 107.8
H-zone NA NA
Crude oil gravity averages around 40o and as of the end of 1987 there were 136 oil wells still producing. The reservoirs are produced under a strong water drive, but there is a component of solution gas drive also. Sidewall core porosity and permeability data from the Senior G&A Operating Co., Kauffman #2, Sec2-T7N-R3E generally run from 250 to 500 millidarcies and 30% to 35% respectively in the productive intervals. Appendix A contains the well log, core locations, and core analysis form the Senior G&A, Kauffman #2.
Six seismic lines, 476 well logs, and 27 side-wall cores were used to determine the origin and structural development of Nebo-Hemphill Field as it relates to the LaSalle Arch; to subdivide the lithostratigraphic Wilcox Group into a genetic sequence stratigraphic framework; and, to reconfirm depositional systems and environments, described in the literature, acting as reservoirs in order to better understand reservoir heterogeneities. Because the Nebo-Hemphill Field lies in an area where the Wilcox Group is dominated by fluvio-deltaic deposition, paleontological data is not available for chronostratigraphic correlations. However, it is still possible to lithostratigraphically correlate the studied interval and make comparisons (figure 52). A biostratigraphic subdivision of the Sabinian Stage using palynomorphs is currently being prepared by William Gregory at LSU. This will enable biostratigraphic correlation of the updip Wilcox Group in the future.
Although 476 wells penetrate the upper parts of the Wilcox Group (figure 2), the data base declines to about 350 wells at the "D" horizon, to 240 wells at the deeper "H" horizon, and to 11 wells that penetrated the underlying Porters Creek Formation. A 1"=2000' basemap of the field is provided as Enclosure 1.
Six reflection seismic lines were used to augment structural and stratigraphic interpretations (figure 3). These cover an expanded region around the well-log study area. Seismic reflectors were tied to well logs by means of a proprietary check-shot velocity survey from a deeply drilled well within the study area.
An amalgamation of ideas from Galloway (1989a), Van Wagoner, et al. (1987), and Posamentier, et al. (1988) was used for ease of description. Galloway's "genetic sequence", bounded above and below by maximum flooding surfaces (MFS), was used as a model because flooding surfaces and MFSs are relatively easy to find and correlate in the subsurface compared with subaerial unconformities. Each genetic sequence represents a progradational pulse into the basin and is composed of three types of systems tracts, a highstand systems tract (HST), a lowstand systems tract (LST), and a transgressive systems tract (TST) (Posamentier, et al. 1988). Van Wagoner, et al. (1987) described the HST to be composed of a progradational parasequence set, LST to be composed of various seafloor fans or aggradational parasequence set, and the Tst to be composed of retrogradational parasequence set.
Structure, cumulative isopach, interval isopach, net sand, sand percent, and selected electrofacies maps as well as various cross-sections were generated. This was done in order to define the depositional systems and environments which made up each genetic sequence. Finally reservoir maps were constructed, based on perforation groupings, in order to determine the size, extent, and controlling factors on the distribution of the oil and gas. Ten structural horizons were picked within the Wilcox using the well log data and mapped for the following horizons from youngest to oldest (figure 4):
1) Top of Wilcox (Top of Carrizo Fm)
2) Top of "A" horizon (Base of Carrizo Fm)
3) Base of Big Shale Mbr (Top "Z" horizon)
4) Top of "B" horizon
5) Top of "C" horizon
6) Top of "D" horizon
7) Top of "E" horizon
8) Top of "F" horizon
9) Top of "G" horizon
10) Top of "H" horizon
Seven structural horizons were picked and mapped using the seismic data for the following reflectors from youngest to oldest (fig. 4):
1) Top of Sparta Fm
2) Top of Wilcox Gp
3) Top of Selma Gp
4) Top of Eagle Ford Fm
5) Top of Glen Rose Gp
6) Top of Sligo Fm
7) Top of Louann Gp.
REGIONAL GEOLOGIC HISTORY
Nebo-Hemphill Field lies on the southern end of the LaSalle Arch which is one of a number of northern rim basement features associated with the opening of the Gulf of Mexico during the Triassic Period (figure 5). The Arch is flanked to the northwest by the North Louisiana Salt Basin, to the northeast by the Central Mississippi Salt Basin, and to the southwest by the Angelina-Caldwell Flexure.
Nunn (1984) showed the Interior Salt Basins occur on thinned continental lithosphere. The Sabine and Monroe Uplifts possibly represent unstretched or only slightly stretched continental lithosphere. If this is correct, the Angelina-Caldwell Flexure, the LaSalle Arch, and the Wiggins Arch also might be interpreted as unstretched or only slightly stretched continental lithosphere. The LaSalle Arch may be analogous to the Wiggins and Sabine arches, which are composed of metamorphic, volcanic, and plutonic rocks formed during the Paleozoic Era (Cagle and Khan, 1983). The arches are associated with the suture zone formed during the Carboniferous Period when Gondwanaland and Laurentia collided to form Pangea. After Pangea broke apart during the Triassic Period, overthrusted igneous and metamorphic terranes, formed during the Carboniferous Period, were left behind as relict terranes.
Jackson and Laubach (1988) showed that the tectonic history of the Sabine Arch was more complex than the result from an association with passive margin subsidence. They summarize possible tectonic explanations for the occurrence of the Sabine Arch into three models: the "Plutonic Dome Model", the "Persistent Buoyant Element Model", and the "Anticlinal Model". These models also indicate the structural development of the LaSalle Arch.
The Plutonic Dome Model was dismissed by Jackson and Laubach (1988) because large gravity and magnetic anomalies are not present, such as the high relief anomalies that occur over the Jackson Dome (figures 6 and 7). These anomalies are believed to have formed as a result of deep plutonic emplacement.
The Persistent Buoyant Element Model calls for the Sabine Arch to act as a relict positive feature from the Paleozoic Era or Jurassic; and, that it subsided less rapidly than adjacent less buoyant crust. Alternatively the Arch originated as a structure that was episodically reactivated and uplifted because it was more buoyant than the adjacent rocks. Detailed stratigraphic analysis implies that the Sabine Arch was not a continuously positive element making this model unacceptable (Jackson and Laubach, 1988).
The Anticline Model allows the Sabine Uplift to act as a positive element, a neutral element, or a negative element. Jackson and Laubach (1988) hypothesize that foreland basin tectonics caused by the Sevier and Laramide orogenic thrusting during the Cretaceous and Tertiary periods might have caused periodic uplift and subsidence of the Sabine Arch. If crustal loading occurring in the Western Cordillera caused the Sabine Arch to uplift, then other basement highs, like the LaSalle Arch, would be effected by the same load (Nunn, pers. comm. 1989) and might show similar periods of uplift and subsidence.
To the south of the LaSalle Arch the Cretaceous Reef Trend occurs. This marks the flexure zone between the stable continental margin to the north, and the unstable continental margin with prolific growth faulting to the south. South of the Cretaceous Reef Trend, the sediments are underlain by extremely thinned continental lithosphere or transitional lithosphere (Buffler et al, 1980).
The lithostratigraphic Wilcox Group in the subsurface of Louisiana was defined by Echols and Malkin (1948) as the first Wilcox sand below the Cane River Formation, the Carrizo Formation, to the last Wilcox sand above the Porters Creek Formation (fig. 8). They subdivided the Wilcox Group into a Lower, Middle, and Upper Wilcox. Echols and Malkin (1948) recognized that the sediments of the Wilcox Group were deposited during an extensive regressive-transgressive cycle.
Sediments of the Middle Wilcox, which are the major reservoir interval within Nebo-Hemphill, were deposited during a period of aerially extensive delta construction, when the Holly Springs Deltaic System in Mississippi and Louisiana prograded into the northern margin of the Gulf of Mexico Basin (figure 9a). Arenaceous, argillaceous, and bituminous (lignitic) lithologies occur at the surface, with interbedded marine clastics downdip (Echols and Malkin, 1948; Galloway, 1968). According to Galloway (1968) the two major deltaic systems, the Holly Springs of Mississippi and Louisiana and the Rockdale of Texas, coalesced downdip. Progradation of the Middle Wilcox extended into the Gulf of Mexico Basin where the interval rapidly thickens.
The upper part of the Wilcox Group of Nebo-Hemphill Field was deposited during a transgressional phase and ended with the regression represented by the Carrizo Formation. The sediments are detrital sands and shales, that are less terriginous and more marine than the Middle Wilcox. Echols and Malkin (1948) noted that the only marine faunas found in the Wilcox Group of central Louisiana and Mississippi are found in the Upper Wilcox.
The depositional systems and electrofacies of the Holly Springs Delta System of Louisiana, Mississippi, and Alabama were described by Galloway (1968). That investigator used the base of the Big Shale as the upper boundary for the lower part of the Wilcox Group. Both Galloway and Echols and Malkin picked the base of the Wilcox Group as the lowest occurring sand above the Midwayan Porters Creek Formation. Galloway suggested the underlying Porters Creek Formation was genetically related to the lower part of the Wilcox Group as the prodeltaic facies of the Holly Springs Delta.
Within the lower part of the Wilcox Group, Galloway recognized four main spatially distributed depositional systems (figure 9b):
1. The Holly Springs Delta System in central Mississippi and Louisiana,
2. The Pendleton Bay-Lagoon System in northwest Louisiana,
3. A Restricted Shelf System in Alabama,
4. An Unnamed Fluvial System cropping out along the flanks of the Mississippi Embayment.
Within Galloway's "Lower Wilcox Group", he defined seven principle electrofacies. They are a bar-finger sand facies, an interdistributary bay mud-silt facies, a distributary channel sand facies, a prodelta mud facies, a distributary mouth bar-delta front sand facies, an interdistributary deltaic plain sand-mud-lignite facies, and a destructional phase sand-mud-lignite facies.
South of the Nebo-Hemphill Field, the Holly Springs Delta System prograded out and over the Cretaceous Shelf Margin. As a result the Wilcox shelf edge extended further into the basin. McCulloh and Eversull (1986) mapped a shale filled canyon system within the Wilcox Group in Avoyelles and St. Landry Parishes of Louisiana lying directly south of Nebo-Hemphill Field (figure 10). Incised canyons may result from fluvial incisement as sea level falls below the shelf edge (Vail et al, 1977) or downdip slope failure followed by retrogressive slumping (Coleman et al, 1983).
Sequence Stratigraphic Framework
Dockery (1986) subdivided the sediments of the Paleogene Period of the northeastern Gulf of Mexico Basin into five successional cycles which correspond to major marine transgressions and regressions. This was based on a study of changes in the amount of speciation of molluscan faunas occurring at the beginning of each cycle, corresponding with a transgression. Each cycle ends with an increased number of extinctions, corresponding with a regression. Dockery interprets these major extinctions as a result of increased selection pressure due to the sea level falling below the shelf edge, causing a reduction in the areal extent of molluscan habitats.
Two of Dockery's cycles include the subsurface Wilcox and Midway Groups (figure 11). The first cycle, encompassing the Midway Group, begins with the transgressive Clayton Formation and ends with the regressive Naheola Formation both of the Midwayan Stage. A prominent disconformity occurs between the Naheola Formation and the overlying Nanafalia Formation all over the Gulf of Mexico's Midwayan shelf. The second cycle, encompassing the Wilcox Group, is represented by the overlying Nanafalia Formation, the Tuscahoma Formation, and the Hatchetigbee Formation of the Sabinian Stage. Dockery shows a prominent disconformity between the Hatchetigbee Formation and the overlying Meridian (Carrizo) Formation of the Claibornian Stage.
In related studies to the northeast of the Holly Springs delta, Hazel et al (1984) used graphic correlation to determine the extent and duration of the unconformities in Alabama, South Carolina, and Virginia for the Paleogene Period (figure 12). This is the region where Galloway (1968) indicated restricted shelf conditions. Stratigraphic sections in Alabama have major unconformities occurring between the Naheola Formation and the Nanafalia Formation and in the middle of the Tallahatta Formation.
In a sequence stratigraphic study of the northeastern Gulf of Mexico, Baum and Vail (1988) recognized seven complete Type 1 and 2 depositional sequences, and portions of two Type 1 depositional sequences, in the Midway, Wilcox, and lower Claiborne groups (figure 13). Type 1 depositional sequences are bounded below by subareal unconformities formed during a sea level fall below the shelf edge and presumably correspond with the major boundaries in Dockery's (1986) and Hazel et al's (1984) works. Type 2 depositional sequences are bounded below by subareal unconformities formed during a relative sea level drop which does not fall below the shelf edge and should not appear in Dockery's or Hazel, et al's works. The boundary between the Maastrichtian and Danian stages (K-T boundary) is represented by a condensed section. This would manifest itself as a downlap surface on a reflection seismic section.
Origin of the LaSalle Arch and General Stratigraphic Effect
The imaged stratigraphy in the region of Nebo-Hemphill Field can be broken into two major intervals: 1) a pre-Triassic Period rift lower interval and 2) a post-Triassic Period upper interval (figures 14, 15, 16, and 17). The post-Triassic Period upper interval can in turn be broken down into three gross seismic stratigraphic groups: A) the base of the Eagle Mills Formation to the top of the Hosston Group, onlap interval, B) the top of the Hosston Group to the middle of the Wilcox Group, downlap interval, and C) the middle of the Wilcox Group to the top of the section incised valley interval (figure 18).
The pre-Triassic Period rift interval is only partially imaged due to seismic energy dispersion in the folded strata. The reflections which are imaged indicate that this interval has been intensely folded, faulted, and eroded. The mound shaped erosional unconformity which lies at 3.2 seconds (2-way travel time) is the most important feature of the pre-rift strata (figures 14 and 15). This erosional unconformity is interpreted as the nose of a Paleozoic thrust. Although not readily apparent at first, the thrust fault has been drawn in where reflectors terminate and the fault is partially imaged as the fault plane flattens out.
The post-Triassic rift interval represents the basin fill of the passive margin of the northern Gulf of Mexico. The entire stratigraphic interval is well imaged and is typified by generally unfaulted and only slightly folded strata with onlap.
Group 1 represents sedimentation during the rift and drift phase of the opening of the Gulf of Mexico (Buffler, 1980). Seismic reflectors are typified by onlap of basinal facies. The only faults in the area are normal faults occurring in the graben just to the north of the erosive thrust fault nose. Eagle Mills redbeds are interpreted to be present in the half-graben (figures 14 and 15). Louann evaporites appear as thin, bedded, and undeformed layers. Two episodes of evaporite deposition are interpreted to have occurred. An early episode which is thickest in the graben and off to the sides of the thrust fault nose thinning considerably over that same nose. A late episode of Louann deposition changes reflector character over the same erosive nose may be a result of changing lithology within the late evaporite deposition in the area. A paleogeographic map showing the graben, thrust fault nose, and distribution of early salt is seen in figure 19.
A generalization of the Louark, Cotton Valley, and Hosston strata is to say that their major centers of deposition were further to the north in the interior salt basins. Moore (1984) has shown this to be true for the Louark Group. Although the normal fault on the north end of the graben does not cut strata higher than the Louann Group, expanded section is still seen over the fault indicating that subsidence in the graben was still a very important factor up until the end of the Jurassic. Palinspastic reconstruction of seismic line P-P' confirms that the largest amount of subsidence occurred in the area of the half-graben for group 1 (figure 20).
Group 2 records early sedimentation after sea floor spreading has presumed to have ceased in the Gulf of Mexico (Buffler, 1980). This records the initial progradational pulses into the study area. Seismic reflectors are typified by downlapping clinoforms and represent a transition zone between terrigenous and marine deposits.
The prograding clinoforms occur on the stable margin and appear relatively flat. However the clinoforms in the middle of the Glen Rose Group show significant aggradation as well as progradation whereas the clinoforms at the base of the Midway Group are shingled which indicates little or no aggradation with very quick progradation. Palinspastic reconstruction of seismic line P-P' and R-R' has shown that greater subsidence occurred to the south and west resulting in expanded stratigraphy in those directions (figures 21 and 22).
Interval isopach maps generally show small amounts of stratigraphic thinning except in the Top Selma to Top Eagle Ford interval isopach map where large amounts of thinning is seen (figure 23). The stratigraphic thinning corresponds with a truncational unconformity seen in the Tayloran Demopolis Formation (figures 14 and 15). This is the only time when the LaSalle Arch experienced relatively major uplift.
Group 3 represents dominantly terrigenous non-marine deposition, the filling of this part of the basin, and finally sediment bypass to the unstable margin south of the Cretaceous Reef trend. Seismic reflectors are typified by incised valley and incised valley fill in the upper Claiborne, Jackson, and Vicksburg Groups (figures 16 and 17). Incised valleys have been described as being associated with sea level dropping below the shelf edge resulting in downcutting by rivers.
The Top Sparta structure map shows a very different picture of the LaSalle Arch in that the western limb of the Arch is greatly reduced (figure 24). The trend of the Arch at lower intervals is north-south and is northwest-southeast on the Top Wilcox structure map (figure 25). By hanging seismic line Q-Q' on the Sparta reflector, the east limb of the LaSalle Arch disappears and only the western limb remains (figure 26).
Wilcox Group Genetic Stratigraphy
The stratigraphic history of the Wilcox Group sediments in Nebo-Hemphill Field over the LaSalle Arch were produced by the interaction of sediment supply, subsidence, and sea level change. Lithostratigraphically the subsurface Wilcox Group extends from the last sand above the Porters Creek Formation, the Naheola Formation, to the first sand below the Cane River Formation, the Carrizo Formation (figure 27). Chronostratigraphically this is believed to represent the upper Midwayan, the Sabinian, and the lower Claibornian stages. Lithostratigraphy was used to subdivide the subsurface Wilcox Group into genetic sequences.
Seismic sequence analysis of the Wilcox and related intervals has shown only one set of shingled prograding clinoforms and no truncational unconformities in the area. The downlap surface is seen on seismic line P-P' at the base of the Porters Creek Formation (figure 28). Posamentier, et al. (1988) have shown that downlap surfaces occur just above the maximum flooding surface of a transgression and is marked by a condensed section.
Well log analysis has revealed four persistant regionally correlatable units within and around the Nebo-Hemphill Field. They are the Porters Creek Formation, an "Unnamed" Shale Member, the Big Shale Member, and the Tallahatta Marl Member (figure 29). The marl member capping the Clayton Tormation, also marking the Porters Creek downlap surface, is not present over the LaSalle Arch but appears off to the flanks and further downdip. The "Unnamed" Shale Member lies above the lowermost cylindrically shaped sand of the Wilcox Group. The Big Shale Member lies above the Z-unit and mards the base of the upper Wilcox. The Big Shale of Louisiana is thought to be correlatable to the Bashi Marl of Alabama. The Tallahatta Member of the Cane River Formation is a thin, 10 feet, marl lying directly on top of the Carrizo Formation over the Arch. On the flanks and further to the south of the Arch, a thin shale member develops between the Tallahata Member and the Carrizo Formation and represents the Recklaw interval in Texas. A second marl, the Weches Formation, also appears above the Tallahatta Formation off of the Arch. A thin, 20 foot, shale break seperates the Weches and Tallahatta formations and represents the distal portions of the Queen City regression. Because each of the four units are regionally correlatable, they are thought to have chronostratigraphic/sequence stratigraphic significance with flooding surfaces near their respective bases.
In a small region, such as this study area, flooding surfaces may only represent local transgressions caused by shifting deltas or by low order cyclic fluctuations in sea level and not actually represent maximum flooding surfaces which record the main transgression. However, because this study area lies in a region which received large amounts of sediment and correlations have been carried beyond the confines of the study area, the low order sea level fluctuations are dampened and major deltaic abandonment can be discounted. Therefore the base of these shale intervals represent flooding surfaces which reflect major transgressions. Based on the recognition of flooding surfaces, the subsurface Wilcox and Midway groups have been subdivided into three genetic sequences (figure 29). It is recognized that the thinner shale intervals, the "Unnamed" Shale Member and the Big Shale Member do not contain actual condensed sections at their flooding surfaces. However, they are correlative with the condensed sections, i.e. time planes marking the point in time where transgression ended and regression began.
Genetic sequence T1, the Midway, extends from the maximum flooding surface at the base of the Porters Creek Formation, to the flooding surface at the base of the "Unnamed" Shale Member (figure 29). It is composed of seismic facies #4 and the lowest part of seismic facies #3 (figure 28). Correlation of the base of the "Unnamed" Shale Member, the Porters Creek Shale, and the regionally ubiquitous cylindrically shaped sand at the top of this genetic sequence is very easy.
Because only 11 wells penetrate the base of the Wilcox and only 4 wells penetrate the Porters Creek Formation, no structural maps were made for these intervals from the well log data. A seismic structure map was generated on top of the Arkadelphia Formation (figure 30). The Porters Creek Formation is a 600 ft (183 m) thick shale unit which appears to be homogenous from the log responses, is part of the highstand systems tract, and represents clastic shelf and prodelta depositional environments. The cylindrically shaped well log response of the sand occurring at the top of the genetic sequence looks similar to a sharp-based shoreface sequence described by Plint (1988) and is interpreted to be the time equivalent of the Naheola Formation in Alabama. Shingled prograding clinoforms and sharp-based shoreface sequences are formed during very fast progradation (Hardenbol, et al, 1977; and Plint, 1988).
Genetic sequence T2, the Holly Springs, extends from the base of the "Unnamed" Shale Member to the base of the Big Shale Member (figure 29). It is composed of the majority of seismic facies #3 excluding the time equivalent of the Naheola Formation at its' base (figure 28). The sand percentage map of the lower part of the Wilcox Group (H-unit) indicate deltaic progradation was from the northeast to southwest (figure 31). Electrofacies and depositional systems from the T2 correspond with those described by Galloway (1968) as formed by a highly constructive highstand systems tract sediment dominated delta. As seen in stratigraphic cross-section E-E' (figure 32), correlation of sand units above the "Unnamed" Shale Member is difficult whereas below that member it was simple.
Isopach and sand percentage maps indicate channels generally oriented northeast to southwest in the middle part of the Wilcox Group. All channel morphologies are strait with the exception of the B-unit which is sinuous. An electrofacies map of the B-unit (figure 33) shows depositional environments to represent a meandering channel, a point-bar, and a cut bank. Productive areas within the B-unit are shaded on the Top B-unit structure map (figure 34). Intervals mapped within the middle part of the Wilcox Group do not appear to incise downward when comparing respective interval isopach and structure maps. As in the lower part of the Wilcox Group, electrofacies seen within each of the units change rapidly laterally, reflecting changing depositional environments (figure 32). Correlation within this interval was made possible only because of the large number of lignites present. Once again electrofacies correspond with those described by Galloway (1968) and represent interdistributary bay/swamp deposits as well as deltaic plane channel deposits, all of which belong in the regressive highstand systems tract. The Z-unit represents deltaic abandonment transgressive systems tract deposits of the Holly Springs delta. An electrofacies map of the Z-unit shows a transgressive shelf sand lying to the southwest of the structural crest of the Arch (figure 35). Productive areas within the Z-unit are shaded on the Base Big Shale structure map (figure 36).
Rock-Eval pyrolysis data from the middle part of the Wilcox Group (figure 37), indicates that the associated organic matter is Type III kerobitumen, or derived predominantly from a terrigenous source. This corresponds with the deltaic plain interpretation of the middle part of the Wilcox Group.
Genetic sequence T3, the Carrizo, extends from the flooding surface at the base of the Big Shale Member to the flooding surface at the top of the Tallahatta Member of the Cane River Formation (figure 29). Only the Carrizo Formation and the A-unit were mapped in the upper part of the Wilcox Group. This was due to a lack of lignite markers in the upper part of the Wilcox Group, like those found in the stratigraphically lower Holly Springs Delta.
Echols and Malkin (1948) reported that the Sabinetown Formation (A-unit) contained the only marine faunas of the Wilcox Group. The isopach map of the A-unit shows a thinning in the southeast quadrant of the study area (figure 38). The sand-percentage map of the A-unit (figure 39) shows average values to be 20%, much less than the underlying middle parts of the Wilcox Group which is about 50%. There is a sand shadow directly to the southwest of the structural trend of the Arch. The fact that the nose of the thrust fault effects deposition in T3 indicates less deposition occurred in area during Genetic Sequence T3 compared with Genetic Sequence T2. Electrofacies in the A-unit are generally 5 to 25 ft (1.5 to 7.6 m) spikes which grade upward into funnel shaped sands and the Carrizo Formation at the top of the subsurface Wilcox Group in the Nebo-Hemphill Field. The A-unit is interpreted as highstand systems tract marine deposits grading upward into Carrizo Formation.
The isopach and sand percentage maps of the Carrizo Formation, and the A-unit, indicate fluvial incisement into the underlying A-unit. An electrofacies map of the Carrizo and underlying related strata from the A-unit shows the distribution of deltaic deposits in relation to the paleo high in the southeast part of Nebo-Hemphill Field (figure 40). Productive areas of the Carrizo Formation are shaded on the Top Wilcox structure map (figure 41). The channel is oriented northeast-southwest and is located in the northwestern quadrant of the study area. Seismically the Carrizo channel system is seen on lines P-P' and R-R' (figures 26 and 28). Electrofacies of the Carrizo Formation are dominantly cylindrically shaped with an occasional funnel shaped pattern at the base. The Carrizo Formation is interpreted as a highstand systems tract fluvial deposits.
The upper part of the Wilcox Group is composed of two seismic facies (figure 28). Seismic facies 1 exhibits medium to medium-high subparallel reflectors which is interpreted as increasing sand content of the upper part of the Wilcox Group, associated with the regression. Seismic facies 2 exhibits medium-low amplitude parallel reflectors interpreted as lower sand content of marine deposition. The boundary between seismic facies 1 and 2 may represent the boundary between the Sabinian and Claibornian stages.
Hazel et al (1984) have shown that there are three recognizable unconformities in the Paleogene Period of the northeast Gulf of Mexico (Alabama). They occur at the base of the Midwayan Stage, at the Midwayan-Sabinian stage boundary, and in the middle of the Claibornian Tallahata Formation.
Dockery (1986) demonstrated two major molluscan extinctions in the same interval and related them to sea level falling below the shelf edge, causing a type 1 subareal unconformity which effectively reduced molluscan habitat. The first unconformity caps the Naheola Formation and the second is at the base of the Meridian (Carrizo) Formation. However, no marine faunas were found in the Meridian and the unconformity could actually lie at the top.
Baum and Vail (1988) have shown five type 1 and three type 2 subareal unconformities and nine condensed section unconformities in the same interval as Dockery and Hazel et al. Although they indicate many sequences, their coastal onlap curves do indicate two larger groupings of depositional sequences which tend to somewhat match the ideas of Hazel et al and Dockery.
Structural Effect on Wilcox Stratigraphy
The structural trend of the LaSalle Arch at the base of the Porters Creek Formation (Top Arkadelphia Formation) is north-south (figure 30). This corresponds to the same as the general trend of the positive gravity anomaly interpreted to be the LaSalle Arch (figure 6). Due to the large influx of sediment in the area during the time of progradation of genetic sequence T2, the Holly Springs, the trend of the LaSalle Arch was modified from north-south to northwest-southeast in the region of the Nebo-Hemphill Field. The shift is seen on structural maps of the top of units G, F, and E of the middle Wilcox (figures 42, 43, and 44). The small narrow trough in the western portion of the study area filled and became a small structural nose at the F horizon and higher. The overall structure of the LaSalle Arch becomes much broader up to the base of the Big Shale Member (figure 36).
The isopach map of the Middle Wilcox (figure 45) indicates an expanded stratigraphic interval west of the structural trend of the LaSalle Arch. No stratigraphic thinning is seen over the paleohigh. Seismic and log analysis do not indicate faulting associated with the expanded interval (figures 28 and 32). The A-unit isopach map (figure 46) shows no expanded section on the west side of the Arch, but there is stratigraphic thinning over the paleohigh. Niether the Middle Wilcox or A-unit isopach maps show expanded section on the east side of the LaSalle Arch. When seismic section Q-Q' is hung stratigraphically on the top Sparta reflector (figure 26), the eastern limb of the LaSalle Arch disappears while the western limb remains.
Since 1940 significant petroleum accumulations have been found in the middle part of the Wilcox Group, genetic sequence T2, of Nebo-Hemphill Field. Minor accumulations have been found in the Carrizo Formation, genetic sequence T3, and even higher intervals. No production has been found as yet in the potential reservoir facies of genetic sequence T1 within the Nebo-Hemphill Field, but production is present a short distance to the north. Gas caps have been found but are not produced due to small reservoir size and marginal economic value. A crude oil sample from the Huffman-Bailey #1 of the E-unit within the middle part of the Wilcox Group was typed geochemically for saturates, aromatics, resins, and asphaltenes. The analysis included C15+ saturated hydrocarbon fraction as well as carbon and sulfur stable isotope analyses. The results of the geochemical analysis are presented in table 2 and graph 1.
Saturates 65.3 API Gravity 41.7
Aromatics 18.2 Sulfur 00.2
Resins 14.8 delta S34 (oil) -00.5
Asphaltenes 01.7 delta C13 (oil) -26.8
Sat./Arom. 03.6 delta C13 (sat) -26.8
C15- fraction 44.3 delta C13 (aro) -25.9
C15+ fraction 55.7 delta C13 (NSO) -25.9
delta C13 (asp) -26.0
Table 2: Analyses of the Huffman-Bailey #1 crude oil. All physical values are expressed as a wt %.
Delta C13 values are relative to the PDB standard (ppm) and delta S34 values are relative to the CDT standard (ppm).
Typical reservoirs in the upper part of T2 are seen in the Z and B-units. The electrofacies map of the Z-unit (figure 35) shows a transgressive shelf sand lying to the southwest of the structural crest of the LaSalle Arch. Productive areas within the Z-unit (figure 36) are parallel to and located over and to the southwest of the structural crest of the Arch. The electrofacies map of the B-unit (figure 33) indicates the interval to be a meandering channel and point bar. Production within the B-unit (figure 34) is generally located within the point bar and in the meandering channel over the structural axis of the Arch. Although electrofacies maps at lower intervals were not constructed, production generally follows similar entrapment patterns in deltaic and lower deltaic plane reservoir facies.
The only reservoir within T3 occurs in the Carrizo Formation, Urania Sand locally. The electrofacies map of the Carrizo Formation and the underlying A-unit (figure 40) shows a Carrizo channel lying over the rift graben and bending around the thrust fault nose lying at depth. Productive areas in the Carrizo Formation are generally located off the main structural trend of the Arch (figure 41) and are not associated with the paleohigh.
Twenty seven sidewall cores from the productive horizons of T2, the Middle Wilcox, were sampled for the purposes of Rock-Eval pyrolysis (table 1). Total Organic Carbon (TOC), Temperature of Maximum Hydrocarbon Generation (Tmax), and a kerogen type indicator as seen through the Hydrogen Index (HI) and Oxygen Index (OI) were measured during this procedure in order to evaluate the source potential for these samples. Rock-Eval pyrolysis and interpretation proceedures are described by Peters (1986).
Sample # TOC(%) Tmax(oC) HI OI Production Unit
Ward #1, Sec40,T7N,R3E
3280 0.45 586 184 68 A
3482 0.33 436 230 57 Z
3484 0.38 575 152 68 Z
3642 0.27 470 192 248 B
3816 0.58 405 181 82 D *
3818 0.38 382 128 139 D
3822 0.37 430 145 243 E
3824 0.64 426 125 170 E *
3852 0.33 507 151 96 E
3876 0.54 428 111 272 E *
3878 0.18 588 177 66 E
3880 0.22 498 136 231 E
3896 0.12 584 175 158 E
3898 0.31 533 129 283 E
3969 1.45 426 178 63 E *
Huffman-Bailey #2, Sec25,T7N,R3E
3643 0.79 432 100 67 B *
3682 0.24 515 141 145 C
3683 0.35 435 168 91 C
3730 0.54 432 124 205 C *
3731 0.58 584 127 128 C *
3799 0.44 431 186 43 D
3802 0.34 424 164 47 D
3804 0.26 434 165 65 D
3816 27.66 420 212 41 D lignite
3820 0.08 532 300 312 D
3842 0.25 472 92 404 E
3843 0.07 471 571 357 E
Table #1: Rock-Eval Pyrolysis Data (* indicates samples with >0.50% TOC)
TOC is measured in order to determine organic richness of the samples. Seven out of the twenty-seven samples had TOC values over 0.5%, the value needed for the proceedure to be valid (Peters, 1986), and are noted with an asterisk (table 1). Tmax is a measure of the samples thermal maturity at which the maximum amount of hydrocarbons were produced during pyrolysis. Average Tmax values for the seven samples with >0.5% TOC is 490oC. If the anomolously high value, 584oC, and the low value, 405oC, are discarded, the average Tmax value is then 429oC which is just below the oil generative window and still immature. By plotting HI versus OI on a modified van Krevelen diagram, kerogen type may be estimated (figure 37). The seven samples with >0.5% TOC displayed HI values averaging 135 mg/g (ranging from 100 to 181 mg/g) and OI values averaging 163 mg/g (ranging from 63 to 284 mg/g). These values plot along the Type III organic matter maturation pathway which is indicative of terrestrially derived organic matter.
The LaSalle Arch in the Nebo-Hemphill area is interpreted from reflection seismic data to be cored by the nose of a Paleozoic thrust fault which was rifted apart during the opening of the Gulf of Mexico. The graben lies directly to the north of the erosive thrust fault nose. Although the graben is seen on only one line, the isopach map of the Sligo to Louann interval and the bow ties, interpreted to be out of the plain, suggest the graben to be oriented west-northwest to east-southeast. This also suggests that extension occurred in a north-northeast to south-southwest direction. Although the Nebo-Hemphill Field makes up only part of the LaSalle Arch, the trend of the Arch is interpreted to be supported by a series of relict continental crust pieces separating the Louisiana and Mississippi Interior Salt Basins, seen as positive gravity anomalies between the basins, then trending to the southeast through Nebo-Hemphill Field.
The structural development of the LaSalle Arch occurred in two steps. The western limb formed due to stratigraphic expansion due to differential subsidence between a less rifted area along the LaSalle Arch trend and a more rifted area to the west and southwest. This is indicated by onlap of basinal facies from the south and west in seismic group 1 and diverging reflectors higher in the section on seismic section Q-Q' and R-R'. The eastern limb of the Arch formed due to regional tilting towards the east sometime after deposition of the Claibornian Sparta Formation. This is seen best when seismic section Q-Q' is hung stratigraphically on the Top Sparta reflector and the eastern limb disappears. Above the Top Sparta reflector, stratigraphic expansion occurs toward the east. Active uplift of the LaSalle Arch happened only once, and that occurred during the Tayloran Stage of the Upper Cretaceous. Therefore uplift of the LaSalle Arch occurred at a different time than uplift on the Sabine Arch (Jackson and Laubach, 1988). Overthrusting in the Western Cordillera, which would predict synchronous uplift (pers. com. Jeff Nunn, 1989), is not a satisfactory mechanism driving basement uplift on the northern Gulf of Mexico rim.
The lower Paleogene Wilcox and Midway groups were broken into three genetic sequences bounded above and below by regionally correlatable flooding surfaces. They are T1 (the Midway), T2 (the Holly Springs), and T3 (the Carrizo) (figure 13 and 30). T1 and T3 were shown by Dockery (1986) to have major subaerial unconformities capping their respective regressive sediments (figure 30) due to sea level fall below the shelf edge. Sea level falls below the shelf edge indicates that the rate of subsidence is less than the rate of eustatic sea level fall and a type 1 subaerial unconformity forms (Posamentier and Vail, 1988). Although the HST of T1 is much less sand rich than T3, they have similar characteristics. Interpreted channel facies within the HSTs of T1 and T3 lie over the rift graben, bend around, and thin over the thrust fault nose lying in the basement. This indicates that the thrust fault nose was a very low order positive feature during the type 1 genetic sequences. It is also noted that even though the thrust fault nose, beneath the Nebo-Hemphill field was a positive feature during these type 1 genetic sequences, the LaSalle Arch did not prevent progradation to the southwest.
Higher frequency fluctuations described by Baum and Vail (1988) (figure 30) are seen within T3, but not T1. The higher frequency fluctuations within T3 appear to represent classic examples of Van Wagoner's (1987) parasequence and not an entire depositional sequence containing a recognizable suite of systems tracts. Dockery (1986) and Baum and Vail (1988) place the type 1 subaerial unconformity at the base of the regionally correlatable laterally continuous Carrizo (Meridian) Formation where a prominent disconformity does exist. If this were true, then the sequence stratigraphic model would predict the Carrizo Formation to be incised valley fill (IVF). The problem with the IVF interpretation is that the Carrizo Formation does not appear to be confined within an incised valley. An alternative interpretation for the Carrizo Formation is that it represents the end of highstand when "vertical stacking grades into lateral migration, resulting in the development of sheet bedding geometry with relatively high lateral continuity" (Posamentier and Vail, 1988). Therefore the type 1 subaerial unconformity would lie towards the top of the Carrizo and not at its' base.
T2 depositional patterns and electrofacies within the study area are those of a highly constructive fluvial dominated deltaic complex (Galloway, 1968). Parasequences as well as interpreted depositional environments exhibit a progradational, or proximal over distal, stacking pattern. A meandering fluvial system, the B-unit, is interpreted to cap the T2 HST. Therefore T2 is interpreted as a type 2 genetic sequence where the rate of subsidence is less than the rate of eustatic sea level fall. The locations of channel facies within T2 are not constrained by the rift graben or the thrust fault nose like the HSTs of T1 and T3. This indicates that the thrust fault nose was not a positive feature during deposition of the T2 HST. The presence of the transgressive shelf sand on the seaward side of the LaSalle Arch's structural trend indicates that the entire Arch, and not just the thrust fault nose, was a positive feature during the T2 TST.
Higher sedimentation rates during the T2 HST modified the structural trend of the LaSalle Arch within the Nebo-Hemphill Field from north-south to northwest-southeast in the direction of progradation. The structural trend of the Arch during the HSTs of T1 and T3 was unmodified. Agradational parasequence stacking patterns, which indicate shelf margin systems tract (Posamentier, et al. 1988), are not present within the study area but may occur further downdip in the expanded Wilcox shelf interval. TST deposits, the Z-unit, onlap directly on top of the T2 HST. Higher frequency fluctuations seen by Baum and Vail (1988) (figure 30) are not recognized within the study area. Many parasequences are seen within T2, but they seem to be more related to delta growth processes.
Migration pathways include deltaic and fluvial channels of the HST, which prograded toward the southwest, and interconnected sands along the trend of the LaSalle Arch. As liquids migrated up dip and encountered the Arch, oil would accumulate in the three-way closure formed by updip pinchouts and the anticline. As initial reservoirs were charged, hydrocarbons would spill under the trap and preferentially follow the structural trend of the Arch.
In order for hydrocarbons to form, like those found in the Nebo-Hemphill Field, sediments must contain sufficient amounts of hydrogen rich organic matter capable of producing liquids when heated over time. The best kinds of organic matter for producing liquid hydrocarbons are predominantly derived from algal rich marine sources and fall along the type I and type II kerogen maturation pathways on modified van Krevelan diagrams. Rock-Eval pyrolisis of sidewall cores from T2 and T3 indicate organic matter to be type III, or predominantly terrigenous phytoclastic material which is hydrogen poor. Even if the organic matter in the study area were of the correct type, Tmax values indicate the organic matter present is still immature. If surrounding rocks could not have generated the crude found in the present reservoirs, the oil must have migrated from mature source areas further down-dip in the direction of progradation, south and southwest (Sassen, in press).
1) The LaSalle Arch basement complex is made up of relict Paleozoic continental crust which was presumably rifted apart during the Triassic Period. Rifting, within T7N,R3E, preferentially occurred to the north of the eroded nose of a Paleozoic thrust fault. Crustal extension occurred in a north-northeast to south-southwest direction.
2) The anticline overlying the basement complex was formed in two steps. The western limb formed syndepositionally by stratigraphic expansion as a result of differential subsidence. Within the lower Paleogene interval, stratigraphic expansion was caused by sediment loading from genetic sequence T2. The eastern limb formed as the result of regional tilting toward the east sometime during deposition of the Claibornian Sparta Formation.
3) Active uplift of the LaSalle Arch only occurred during the Upper Cretaceous Tayloran Stage where an angular unconformity is observed on reflection seismic data. Because uplift timing is not synchronous for the LaSalle and Sabine arches, overthrusting in the Western Cordillera could not be the mechanism for uplift.
4) The lithostratigraphic Wilcox Group of central Louisiana centered around Nebo-Hemphill Field, was subdivided into three genetic sequences: T1 (Midway), T2 (Holly Springs), and T3 (Carrizo). The HST of T1 and T3 (type 1 genetic sequences) are interpreted to have been deposited during a relative fall in sea level. This resulted in similar homogenous wave dominated deltaic systems capped by braided stream systems. T1 and T3 did not modify the structural trend of the Arch. The HST of T2 (a type 2 genetic sequence) is interpreted to have been deposited during a relative rise in sea level. This resulted in heterogenous fluvial dominated deltaic systems. T2 greatly modified the structural trend of the LaSalle Arch.
5) The LaSalle Arch acted as a very low positive feature with relief of probably only a few meters during T1 and T3, type 1 genetic sequences, and had no relief during T2, a type 2 genetic sequence. Although the thrust fault nose and rift graben controlled the location of channel facies within T1 and T3, the LaSalle Arch itself did not prevent deltaic sedimentation to the southwest in any of the progradational events.
6) Pyrolysis of sidewall core samples from T2 has shown the kerobitumen to be type III and thermogenically immature. Because type III kerobitumen is terrestrially derived and hydrogen deficient, crude oil found within the Wilcox reservoirs of Nebo-Hemphill Field could not have come from the resident organic matter.
I wish to gratefully acknowledge and thank EXXON, U.S.A. and its' employees Bill Drennan, Mark Solien, and Niel Sammis for the use of reflection seismic data and overall help with this research. Without their help, a major portion of this study could not have been accomplished. Jeff Nunn provided many helpful comments regarding Gulf of Mexico basement features, and theoretical advice about rifting and crustal loading. Partial support for this research was provided by the Basin Research Institute at Louisiana. Final thanks go to Greg Riley, Mark Pasley, and Bill Gregory for reviewing various parts of the manuscript and for putting up with my ideas.
Baum, G. R., and Vail, P. R., 1988, Sequence Stratigraphy, Allostratigraphy, Isotope Stratigraphy and Biostratigraphy: Putting it all Together is the Atlantic and Gulf Paleogene. GCSSEPM Foundation Eighth Annual Research Conference, 1988, PP.15-23.
Buffler, R. T., et al, 1980, Structure and Early Geologic History of the Deep Central Gulf of Mexico Basin. Proceedings of a symposium at Louisiana State University, Rex H. Pilger (ed.), March 3-5, 1980, pp.3-16.
Bulling, T.P. and Breyer, J.A., 1989, Exploring for Subtle Traps with High-Resolution Paleogeographic Maps: Reklaw 1 Interval (Eocene), South Texas. AAPG Bulletin, V. 73, No. 1, p.24-39.
Cagle, J. W., and Khan, M. A., 1983, Smakover-Norphlet Stratigraphy, South Wiggins Arch, Mississippi and Alabama. Transactions, GCAGS, Vol. 33, pp.23-29.
Coates, E. J., et al, 1980, Subsurface Wilcox Lignite in West-Central Louisiana. Transactions: GCAGS, Vol. 30, pp.309-332.
Cross, T. A., and Lessenger, M. A., 1988, Seismic Stratigraphy. Ann. Rev. Earth Planet, Sce. 1988, 16:319-54.
Dockery, III, D. T., 1986, Punctuated Succession of Paleogene Mollusks in the Northern Gulf Coastal Plain. Palaios, 1986, Vol. 1, pp.582-589.
Echols, D. J., and Malkin, D. S., 1948, Wilcox (Eocene) Stratigraphy, A Key to Production. AAPG Bulletin, Vol. 32, No. 1, pp.11-33.
Galloway, W. E., 1968, Depositional Systems of the Lower Wilcox Group, North-Central Gulf Basin. Transactions: GCAGS, Vol. 18, 1968, pp.275-289.
Galloway, W. E., 1989, Genetic Stratigraphic Sequences in Basin Analysis I: Architecture and Genesis of Flooding-Surface Bounded Depositional Units. AAPG Bulletin, Vol. 73, No. 2, pp.125-142.
Galloway, W. E., 1989, Genetic Stratigraphic Sequences in Basin Analysis II: Application to Northwest Gulf of Mexico Cenozoic Basin. AAPG Bulletin, Vol. 73, No. 2, pp.143-154.
Glawe, L.N., 1989, Stratigraphic Relationships Between Odontogryphaea Thirsae Beds and the Big Shale of the Wilcox (Paleocene-Eocene) in Louisiana. Transactions, GCAGS, Vol.34, pp.375-383.
Haq, B. U., Hardenbol, J., and Vail, P. R., 1987, Chronology of Fluctuating Sea Levels Since the Triassic. Science, Vol. 235, 6 March 1987, pp.1156-1167.
Hazel, J. E., 1984, Significant Unconformities and the Hiatuses Represented by them in the Paleogene of the Atlantic and Gulf Coastal Province. AAPG Memoir 36, pp.59-66.
Jackson, M., and Laubach, S. E., 1988, Sabine Arch: Cretaceous and Tertiary Compressional Tectonics as the cause of the Sabine Arch, East Texas and Northwest Louisiana. Transactions, GCAGS, Vol. 38, pp.245-256.
Lawless, P.N., 1989, The Effect of the LaSalle Arch on Wilcox Group Stratigraphy. An unpublished Master's Thesis at Louisiana State University, 204 pages.
Lawless, P.N., 1989, Origin and Structural Development of the LaSalle Arch, LaSalle Parish, Louisiana. ABS. AAPG Bulletin, May 1990.
Lawless, P.N., 1989, Lower Paleogene Genetic Sequence Stratigraphy, Nebo-Hemphill Field, LaSalle Parish, Louisiana. ABS. AAPG Bulletin, May 1990.
Lawless, P.N., and Hart, G.F., 1990, The LaSalle Arch and Its' Effect on Lower Paleogene Genetic Sequence Stratigraphy, North-Central Louisiana, Transactions, GCAGS, Vol.35, in press.
Lowry, P., Lemoine, R., and Moslow, T., 1986, Sedimentary Facies of the Uppermost Wilcox Shelf-Margin Trend, South-Central Louisiana, SEPM Core Workshop 9, pp. 363-412.
Loutit, T.S., Hardenbol, J., Vail, P.R., and Baum, G.R., 1988, Condensed Sections: The Key to Age Determination and Correlation of Continental Margin Sequences. SEPM Special Publication #42, edited by: Wilgus, C.K., Hastings, B.S., Kendall, C.G.St.C., Posamentier, H.W., Ross, C.A., and Van Wagoner, J.C., pp.184-213.
McCulloh, R.P., and Eversull, L.G., 1986, Shale-Filled Channel System in the Wilcox Group (Paleocene-Eocene), North-Central South Louisiana. Transactions, GCAGS, Vol. 36, pp.213-218.
Mitchum, Jr., R.M., Vail, P.R., and Thompson, III, S., 1977, Seismic Stratigraphy and Global Changes of Sea Level, Part 2: The Depositional Sequence as a Basic Unit for Stratigraphic Analysis. AAPG Memoir #26, edited by: Payton, C.E., pp.53-62.
Mitchum, Jr., R.M., Vail, P.R., and Thompson, III, S., 1977, Seismic Stratigraphy and Global Changes of Sea Level, Part 6: Stratigraphic Interpretation of Seismic Reflection Patterns in Depositional Sequences. AAPG Memoir #26, edited by: Payton, C.E., pp.117-133.
Nunn, J.A., et al, 1984, Thermal Evolution of the North-Central Gulf Coast. Tectonics, Vol. 3, No. 7, pp.723-740, 1984.
Peters, K.E., 1986, Guidelines for Evaluating Petroleum Source Rock Using Programmed Pyrolysis, AAPG Bulletin, Vol. 70, No. 3, pp. 318-329.
Posamentier, H.W., et al. 1988, Eustatic Controls on Clastic Deposition I - Conceptual Framework. SEPM Special Publication #42, edited by: Wilgus, C.K., Hastings, B.S., Kendall, C.G.St.C., Posamentier, H.W., Ross, C.A., and Van Wagoner, J.C., pp.
Posamentier, H.W., and Vail, P.R., 1988, Eustatic Controls on Clastic Deposition II - Sequence and Systems Tract Models. SEPM Special Publication #42, edited by: Wilgus, C.K., Hastings, B.S., Kendall, C.G.St.C., Posamentier, H.W., Ross, C.A., and Van Wagoner, J.C., pp.125-154.
Sassen, R., 1987, Organic Geochemistry of the Salt Dome Cap Rocks, Gulf Coast Basin, in: Dynamical Geology of Salt and Related Structures: I. Lerche and J.J. O'Brien, eds., Academic Press, pp. 163-175.
Sassen, R., in press, Lower Tertiary and Upper Cretaceous Source Rocks in Louisiana and Mississippi: Implications to the Gulf of Mexico Crude Oil.
Scardina, A.D., 1982, Tectonic Subsidence History of the North Louisiana Salt Basin, LSU Publications in Geology and Geophysics, Gulf Coast Studies, Vol. 2, 1982.
Serra, O., editor, 1985, Sedimentary Environments from Wireline Logs. a Schlumberger Technical Publication.
Snedden, J.W., 1984, Validity of the use of the Spontaneous Potential Curve Shape in the Interpretation of Sandstone Depositional Environments. Transactions, GCAGS, Vol. 34, pp.255-263.
Vail, P.R., 1987, Seismic Stratigraphy Interpretation Using Sequence Stratigraphy, Part One: Seismic Stratigraphy Interpretation Procedure. Atlas of Seismic Stratigraphy, Volume 1, edited by Bally, A.W., pp.1-10.
Van Wagoner, J.C., Mitchum, Jr., R.M., Posamentier, H.W., and Vail, P.R., 1987, Part 2: Key Definitions of Sequence Stratigraphy. in: AAPG Studies in Geology #27: Atlas of Seismic Stratigraphy, Vol. 1, edited by: Bally, A.W.
Winker, C.D., 1982, Cenozoic Shelf Margins, Northwestern Gulf of Mexico. Transactions, GCAGS, Vol. 32, pp.427-448.
Sidewall Core Analysis of the Senior G&A, Kauffman #2
Sec2,T7N,R3E Nebo-Hemphill Field LaSalle Parish, Louisiana