BASIN ANALYSIS AIDS EXPLORATION IN THE MISSISSIPPI INTERIOR SALT BASIN

Based on a workshop sponsored by PTTC's Eastern Gulf Region on July 21, 1999, in Jackson, MS.

BOTTOM LINE

Applying basin analysis and petroleum system modeling to the Mississippi Interior Salt Basin has the potential to significantly improve the success rate of exploration wells.

PROBLEM ADDRESSED

The northeastern Gulf of Mexico has been explored primarily by small- and medium-sized companies that typically do not have the resources to analyze the vast amount of available data needed to perform basin analysis. The University of Alabama has recently conducted a comprehensive analysis of the Mississippi Interior Salt Basin (MISB) to demonstrate the value of integrated basin analysis. The results have been made available in terms of the MISB model and its implication for exploration potential in this region.

KEY WORDS:

Basin Analysis, Mississippi Interior Salt Basin, Burial and Thermal Maturation Modeling

SPEAKERS

Tectonic and Depositional Setting of MISB
Mark Puckett, University of Alabama

Burial and Thermal History Modeling
Ernest A. Mancini, University of Alabama

Petroleum Trends in MISB
Bennett Bearden, University of Alabama

TECHNOLOGY OVERVIEW

The northeastern Gulf of Mexico remains underexplored, containing numerous sedimentary basins with many formations that have a high potential for hydrocarbon accumulation. In this area, small- and mediumsized companies drill most of the exploration wells. These companies do not have the resources to perform basin analysis or to synthesize the large amount of publicly available data. Recently, the Center for Sedimentary Basin Studies at the University of Alabama undertook a study to delineate the petroleum plays, subplays, and trends, and to increase the amount of available data through a comprehensive analysis of the MISB.

Tectonic and Depositional Setting
The MISB is the largest of a series of continental margin basins associated with the opening of the Gulf of Mexico. It includes areas in east Texas, Louisiana, Mississippi, southwestern Alabama, and offshore Florida. The basin’s depositional and stratigraphic summary provides the foundation for predicting hydrocarbon source, migration path, seal and reservoir play distribution. Basin-scale stratigraphic summaries block out a blueprint for large-scale trends that may be useful for predicting changes in depositional setting, lithology, and texture when logs or core are not available.

Regional- to basinal-scale structural studies are also critical for understanding distribution of major uplifts, downwarps, salt structures, basement ridges, and fault zones. These are important because they may control migration of basinal fluids (including hydrocarbons), form updip seals to reservoirs, and remove or thicken stratigraphic intervals. Structural and depositional models provide the fundamental data required for more advanced studies such as thermal history modeling and predicting trends of hydrocarbon accumulation.

Burial and Thermal History
Modeling Burial History. Burial-history modeling is crucial in determining the generation, migration, and preservation of hydrocarbons in the basin. Since it is dependent upon a sound regional model for the basin's tectonic and depositional history, the magnitude of depositional and erosive events are critical to interpreting the burial and thermal history.

The MISB's burial history was based on interpreting five regional cross sections containing 48 key wells. The basin history of each well was determined using BasinMod ® software. Information interpreted from the cross sections, well logs, and biostratigraphic study included the geologic ages of selected horizons, water depth, stratigraphic thickness, lithology, sediment accumulation and subsidence rates, unconformities and faulting.

Sediment column thickness was corrected for compaction using the Sclater and Christie (1980) method. Key stratigraphic horizons were determined through well log study and BasinMod software. Sediment accumulation and subsidence rates were also determined using BasinMod software.

Using BasinMod, the mean stratigraphic thickness for five key intervals were determined: Jurassic (4,746 ft), Lower Cretaceous (6,242 ft), Upper Cretaceous (3,858 ft), lower Tertiary (4,989 ft) and upper Tertiary (2,926 ft). Mean sandstone sediment accumulation rates range from 311/ my for Lower Cretaceous sandstones to 170 ft/ my for Jurassic sandstones. Mean shale sediment accumulation rates range from 108 ft/ my for Upper Cretaceous clays to 90 ft/ my for lower Tertiary shales. Mean limestone sediment accumulation rates range from 122 ft/ my for Jurassic limestones to 57 ft/ my for Upper Cretaceous chalks. Mean anhydrite sediment accumulation rates are 85 ft/ my for Lower Cretaceous anhydrites. Mean tectonic subsidence rates for the intervals are Jurassic: 130 ft/ my; Lower Cretaceous: 72 ft/ my; Upper Cretaceous: 46 ft/ my; and lower Tertiary: 45 ft/ my.

Thermal History. Thermal history modeling, which builds on burial history modeling, is crucial to determining whether a basin has hydrocarbons in commercial quantities and whether they are oil, natural gas, or both. Information critical to thermal analysis includes determining present-day heat flow and paleoheat flow and thermal conductivity, as well as the amount and type of kerogen and timing of heating events.

The five regional cross sections with 48 key wells provided the basis for interpretation of thermal history. Each well was subjected to BasinMod analysis. Input data included bottom-hole temperature, present-day geothermal gradient, present day heat flow, vitrinite reflectance, thermal alteration, Tmax, paleogeothermal gradient, paleoheat flow, thermal conductivity, total organic carbon and kerogen type.

Thermal history analysis indicated that effective source rocks in the MISB include Upper Jurassic Smackover carbonate mudstones throughout the basin and Upper Cretaceous Tuscaloosa shales in the south central part of the basin. Where appropriate organic facies are present, Lower Cretaceous source rocks are possible in the south central portion of the basin. Tertiary shales have not generated hydrocarbons. Smackover samples from the lower and middle carbonate mudstones average 0.81% total organic carbon, with maximum values as high as 2.52% in thermally mature carbonate mudstones.

A hydrocarbon maturation and generation trend can be observed, based on the thermal maturation profiles for wells in the study area. In much of the basin, the generation of hydrocarbons from Smackover carbonate mudstones initiated at 8,000-11,000 ft during the Early Cretaceous and continued into the Tertiary. Locally, hydrocarbon generation began at 7,000-8,000 ft from Tuscaloosa shales during the Tertiary in portions of the Perry sub-basin. Near the Jackson Dome, hydrocarbon gas generation begins at 15,000 ft.

Petroleum Plays Identified
Three major petroleum plays were identified:

The basement ridge play is located updip of the regional peripheral fault trend where the Jurassic Louann Salt is thin or absent. This play formed in association with pre-Jurassic basement paleotopography. Traps in this subplay are characteristically anticlines, faulted anticlines that developed over basement highs and combination traps (e. g. porosity or permeability termination on the flanks of anticlines). Within this play, the Upper Jurassic shallow-marine carbonate and sandstone subplay can be recognized. All reservoirs in this subplay are in the Smackover Formation.

Underdeveloped potential reservoirs in the basement ridge play include the Upper Jurassic Norphlet, Cotton Valley and Haynesville Formations and the Lower Cretaceous Hosston Formation. Seismic and well log interpretations indicate that Norphlet traps may occur where the Norphlet either thins or pinches out over or against basement highs. The Smackover also remains somewhat underexplored. There also is potential in the updip areas north of the regional peripheral fault trend in and around Holmes Co., Mississippi and in a reef/ shoal play along the northern extent of the MISB.

The Regional Peripheral Fault Trend Play is located basinward of the updip limit of Louann Salt. The petroleum play along this fault trend is created by saltrelated features associated with the Pickens, Gilbertown, Melvin, and West Bend Fault systems that tend to be located parallel to the MISB margin. The faults are subparallel to the regional strike, associated with grabens 5-8 miles across, and correspond to the updip limit of thick Jurassic salt. Basinward withdrawal of salt from its updip limits and downward flexure of overlying units is believed to have caused the regional peripheral fault trend.

There are two major subplays: the Norphlet fluvial and aeolian sandstone, and the Upper Jurassic shallowmarine carbonate and sandstone. Prospective underdeveloped reservoirs in the regional peripheral fault trend play include Upper Jurassic Hanynesville and Cotton Valley formations, the Lower Cretaceous Hosston, Sligo, Pine Island, Rodessa, Paluxy, Fredericksburg, and Washita formations and the Upper Cretaceous Tuscaloosa, Eutaw and Selma strata.

The MISB (Salt Anticline) Playis the largest of several sedimentary basins along the northern margin of the Gulf of Mexico region. Encompassing 6,000 square miles that was an actively subsiding depocenter throughout the Mesozoic and early Cenozoic, its structural boundaries include 1) the regional peripheral fault trend to the north, 2) the Mobile graben to the east, 3) the Wigins arch complex-Adams Co. high on the south/ southwest, 4) the La Salle arch on the west, and 5) the Late Cretaceous Monroe Uplift to the northwest. It is the most petroliferous basin in the Eastern Gulf of Mexico.

Basins and uplifts in the northern Gulf Coastal Plain. Modified from Pilger (1981).

MISB is characterized by an updip- to-downdip progression of salt-related features including peripheral salt ridges, low relief salt pillows, intermediate anticlines, high-relief anticlines, turtle structures, and piercement domes. Traps are structural and combination traps associated with salt tectonism. Key reservoirs in the basin are developed in Smackover, Norphlet, Cotton Valley, Tuscaloosa and Eutaw strata. Eleven subplays can be identified within the MISB.

LESSONS LEARNED

Modeling of the MISB indicates a basinward progression of structures that include 1) updip basement ridges, 2) updip peripheral listric, fault trend coincident with termination of salt (peripheral salt ridge, 3) low relief salt pillows, 4) salt anticlines, 5) salt diapirs, and 6) post-diapiric structures. This model for the MISB is easily transferred to other Gulf basins.

The key to continued successful exploration in the MISB for Mesozoic and Cenozoic reservoirs will be delineating traps associated with salt movement and recognizing favorable lithofacies. Basement highs updip of the basin proper also will be prospective. Drilling of salt dome flanks may be the most important trend in renewing exploration of Cretaceous strata.

Sequence stratigraphic analysis, in combination with depositional modeling, can be used to identify potential reservoir, source, and seal beds, as well as identifying stratigraphic pinchouts and unconformity traps. Such analysis has great potential to predict lithofacies distribution and geometry in the MISB. This is important because the region is located where transgressive systems tracts and associated condensed sections have been proven to provide the principal petroleum source rocks, while the highstand systems tract deposits constitute the primary petroleum reservoir rocks.

CONNECTIONS:

Mark Puckett, Stratigrapher
University of Alabama
Box 870338, 202 Bevill Bldg.,
Tuscaloosa, AL 35487
Phone 205-348-7167, Fax 205-348-0818 E-mail mpuckett@wgs.geo.ua.edu

Bennett Bearden, Petroleum Geologist
University of Alabama
Box 870338, 202 Bevill Bldg.,
Tuscaloosa, AL 35487
Phone 205-348-4319, Fax 205-348-0818 E-mail bbearden@wgs.geo.ua.edu

For information on PTTC’s Eastern Gulf Region and its activities contact:
Ernest A. Mancini, Professor of Geology University of Alabama Box 870338, 202 Bevill Bldg., Tuscaloosa, AL 35487
Phone 205-348-4319, Fax 205-348-0818, E-mail emancini@wgs.geo.ua.edu

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