Steven D. Mann

Geologic Framework of Norphlet and Pre-norphlet Strata of the Onshore and Offshore Eastern Gulf of Mexico Area

ABSTRACT Hydrocarbon accumulations have been discovered in the Jurassic Norphlet Formation in the onshore and offshore eastern Gulf of Mexico area. An understanding of the regional geologic framework of the Norphlet and pre-Norphlet stratigraphic succession in the study area is crucial to successful exploration for and development of Norphlet hydrocarbon reservoirs in the eastern Gulf region. These strata include Jurassic sedimentary rocks and pre-Jurassic sedimentary and crystalline rocks. Of these strata, only the Norphlet Formation has produced economic quantities of hydrocarbons from reservoirs which include eolian dune, interdune, wadi, and marine sandstones. However, the pre-Norphlet geology and paleotopography of the area controlled, to a large degree, Norphlet depositional patterns and subsequent structural development. Especially important were the locations of pre-Jurassic paleohighs and the distribution and thickness patterns of the underlying Jurassic Louann Salt. Norphlet hydrocarbon traps are generally associated with structures that have resulted from halokinesis of the Louann. Traps include salt anticlines, faulted salt anticlines, and extensional faults associated with salt movement. In addition to these salt-related structural traps, traps resulting from pre-salt basement paleotopography may exist in the study area. These traps might include Norphlet structural highs associated with preexisting basement highs and Norphlet stratigraphic terminations along the flanks of basement highs. Where Norphlet reservoir facies are present, the key factor controlling hydrocarbon accumulation potential is structural setting. In the Mississippi-Alabama-Florida (MAFLA) region that is underlain by the Norphlet Formation, four structural settings have been identified. Types A and B structural settings are characterized by Norphlet sediments overlying basement or thin salt; no Norphlet fields have been established in these settings. Types C and D structural settings are characterized by structures related to movement of thick Louann Salt; all Norphlet fields established in the MAFLA region occur in these settings. Four geographic regions have been defined in the study area for the purpose of characterizing Norphlet petroleum potential. Region 1 has excellent potential for future Norphlet discoveries, Region 2 has low to moderate potential, and Regions 3 and 4 have very little to no potential.

Controls on Reservoir Development in a Shelf Carbonate: Upper Jurassic Smackover Formation of Alabama

Hydrocarbon reservoirs of the Upper Jurassic Smackover Formation in Alabama are predominately oolitic and pelletal dolostone. Pore systems are dominated by moldic and secondary intraparticle pores, intercrystalline pores, or mixtures of these pore types. All Smackover reservoirs in Alabama have been strongly affected by early cementation, dissolution of calcium-carbonate allochems, and dolomitization. Marine-phreatic cement occluded primary interparticle porosity in much of the Smackover reservoirs in Alabama. Dolomitization of the Smackover in Alabama included penecontemporaneous, early burial, and late (deep) burial episodes. Early burial dolomite predominates. Fabric-selective dolomitization yielded reservoirs strongly influenced by both depositional fabric and diagene is. Nonselective dolomitization yielded reservoirs with intercrystalline pore systems shaped primarily by diagenesis. Porosity evolution was controlled regionally by level of thermal exposure, mode of dolomitization, and proximity to the Wiggins arch. Thermal exposure is inversely related to porosity, but the relationship is weak (r2 < 0.5). Fabric-selective dolostone is slightly more porous than nonselective dolostone when averaged over the entire study area (averages of 18.1% and 15.1%, respectively; p = 0.0001), but nonselective dolostone is more porous at a given level of equivalent vitrinite reflectance. Smackover fields on the north flank of the Wiggins arch are unusually porous given their level of thermal maturity, and are unusual in other ways as well. Local porosity variation was controlled by depositional fabric, early cementation, dissolution, and burial compaction and cemen ation. Regional permeability variation cannot be explained using existing data. Permeability is locally controlled by pore-throat size, the effects of dolomite crystal-size distribution, early cementation, fracturing, and burial compaction and cementation. Pore-throat size exhibits the strongest overall correlation with permeability (r2 = 0.54). Permeability and porosity are strongly correlated locally, but the regional correlation is weak.

Classification of Lithified Carbonates Using Ternary Plots of Pore Facies: Examples from the Jurassic Smackover Formation

Ternary diagrams whose apexes are carbonate pore types (ternary pore plots) are used to summarize quantitative data derived from point counting of thin sections, using a modification of the genetic carbonate-rock porosity classification of Choquette and Pray (1970). Ternary pore plots provide information on the shapes and origins of pore-system elements. Hence, ternary pore plots complement engineering data, which give information on the sizes of pore-system elements. Thin-section point-count data are inexpensive and easy to collect, and can be used to guide more expensive engineering analyses.

Subaqueous Evaporites of the Buckner Member, Haynesville Formation, Northeastern Mobile County, Alabama

ABSTRACT The lower part of the Buckner Anhydrite Member of the Haynesville Formation (Upper Jurassic) was deposited as shallowing upward cycles of subaqueous to subaerial deposits on the north flank of the Wiggins Arch in northeastern Mobile Country, Alabama. The unit studied conformably overlies the Smackover Formation and is generally evaporite dominated. The Buckner Anhydrite averages about 35 m (115 ft) in thickness and has been buried to depths of 5.5 km (> 18,000 ft). In spite of this deep burial, it has suffered little deformation since alteration of gypsum to anhydrite. Subaqueous evaporites in the Buckner are dominated by bedded massive, bedded mosaic, bedded nodular, bedded nodular mosaic, and mosaic structural types. Buckner cycles begin with the selenite facies in which bedded nodular mosaic and bedded mosaic structural styles often display a preferential vertical alignment of anhydrite nodular pseudomorphs after gypsum. Selenite crystals were precipitated on the sediment surface possibly below wave base but within the photic zone. Gypsarenite, which vertically succeeds selenite, is a laminated deposit in which ripples and low-angle cross-beds indicate clastic deposition above wave base. The upper part of this facies may have been subaerially exposed. Stromatolite domes formed in relatively quiet water areas. Subaqueous evaporites of this unit commonly grade upward into a generally poorly laminated anhydrite often containing burrow structures, detrital silt, and possible root structures. This may have been a gypsite facies which formed in the zone of soil moisture. Often this facies is located stratigraphically just below probable storm deposits and tidal flat-sabkha deposits which accumulated under subaerial conditions. These deposits are located at the top of the cycle. Subaerial conditions are indicated by desiccation cracks, an influx of oxidized red silt and clay, and erosional scours. There probably was not a significant change in water depth at the transition from the Smackover Formation to the Buckner Anhydrite. Subaqueous Buckner evaporites were deposited conformably over subtidal deposits at the top of the Smackover. The formation of microcrystalline dolomite at the top of the Smackover probably occurred below the sediment/water interface. Relative thickness of cycles, facies correlations, and the frequency and duration of subaerial exposure indicate the relative paleotopograpic position of various locates in the area. Buckner cycles were controlled by the rate of evaporite aggradation versus sea level rise and/or rate of subsidence.

Depositional History of the Smackover-Buckner Transition, Eastern Mississippi Interior Salt Basin

ABSTRACT Shoaling upward cycles of the upper part of the Smackover Formation are locally capped by thin sabkha deposits adjacent to and on the crests of paleotopographic highs in the eastern Mississippi interior salt basin (MISB). These sabkhas are overlain by peritidal carbonates of the uppermost Smackover. Relative sea level fell during late-Smackover progradation and sabkha deposition, then rose again to deposit peritidal carbonates. A sharp contact typically separates these peritidal carbonate deposits and the overlying massive Buckner Anhydrite in the MISB. Evidence for a significant sea-level change at the Smackover-Buckner Boundary is lacking. Smackover sabkha deposits in the MISB consist of intercalated matrix-dominated nodular anhydrite and anhydritic dolostone in which the proportion of anhydrite commonly increases upward. They are overlain by micritic, pelletal, or oolitic peritidal dolostone. Smackover sabkhas formed on local paleotopographic highs concentrated along a north-south trending ridge produced by movement of the Louann Salt. Because salt movement was variable in timing and intensity causing Smackover sabkhas to be discontinuous and probably not everywhere the same age. The carbonates that overlie the Smackover sabkhas resemble typical Smackover carbonates and include reservoir strata. Massive Buckner Anhydrite strata are predominantly subaqueous saltern deposits, though peritidal evaporite deposits occur on the margins of the MISB. Saltern deposits formed in a gypsum-precipitating lagoon in the eastern MISB. This region was a silled basin, sheltered from less saline water in the main part of the MISB by a salt-cored anticline capped by carbonate grainstone shoals in eastern Mississippi. Saltern deposits are dominated by selenite and gypsarenite. Saltern deposits are locally interbedded with subtidal hypersaline to normal-marine carbonate deposits which may record sea-level fluctuations or changes in water circulation. A saltern, or evaporite lagoon, formed in the eastern MISB at the beginning of Buckner time as a result of restriction of water influx into the eastern MISB and resultant rapid increase in salinity to gypsum saturation. The salinity increase was a chemical event; hence the base of the massive anhydrite is a time plane, and its extent approximates that of the Buckner evaporite lagoon. Subaqueous evaporites of the basal Buckner occupy a smaller area than do peritidal carbonates of the uppermost Smackover, suggesting that evaporative drawdown was a contributing factor in the deposition of the massive anhydrite. Smackover sabkhas are not physically connected to the subaqueous evaporite deposits of the Buckner nor did they form in the same way. Saltern deposits thicken away from paleotopographic highs and have a blocky density-log pattern. Smackover sabkhas typically have spiky density-log patterns and thin away from paleotopographic highs.

Regional Variation in Microscopic and Megascopic Reservoir Heterogeneity in the Smackover Formation, Southwest Alabama

ABSTRACT Quantitative (rank) measures of microscopic and megascopic reservoir heterogeneity are used to characterize the distribution of reservoir heterogeneity in Smackover hydrocarbon fields and wildcat wells in southwest Alabama. Microscopic reservoir heterogeneity (µH) is {[(0.25) + (meanlnK) + (1.5lnK)]/3}. Megascopic Heterogeneity (MH) is [(# of reservoir intervals) + (# of high-K reservoir intervals) + ( of # of reservoir intervals)] where reservoir rock is defined as exhibiting permeability values >= 0.1 md and high-permeability reservoir rock exhibits permeability values >= 1.0 md. Both MH and µH are determined from core data. The Dykstra-Parsons coefficient (DP) is a measure of microscopic heterogeneity that is partially independent of µH(r2 = 0.428). All three of these parameters are primarily measures of vertical heterogeneity, although averaging of wells within a field incorporates lateral heterogeneity in µH and DP. µH and MH are distributed in contrasting but related patterns. µH generally decreases from northwest to southeast, with the highest values found in the vicinity of the Choctaw Ridge complex north of the Mississippi Interior Salt Basin (MISB). Moderately high values typify the Manila Embayment and the Conecuh Ridge complex to the south, whereas lower values are found in the MISB, on the north flank of the Wiggins Arch, and in the Conecuh Embayment. µH values are high in the Moldic Pore Facies and low in the Intercrystalline Pore Facies. The distribution of MH is roughly opposite to that of µH. MH values are high on the north flank of the Wiggins Arch, on the Conecuh Ridge complex and in the Conecuh Embayment: MH values are low near the Choctaw Ridge complex. The Conecuh Ridge is unique because it is characterized by high values of both µH and MH. Also, the low-relief north-south trending salt-cored anticline in western Washington County is characterized by relatively high values of MH. Reservoirs belonging to the Moldic Pore Facies tend to be homogeneous with respect to MH, whereas reservoirs assigned to the Intercrystalline Pore Facies are characterized by relatively high values of MH. MH and µH vary congruently with pore-system characteristics (controlled by depositional patterns, dissolution, and dolomitization) and regional structural and paleogeographic trends. This suggests that reservoir heterogeneity characteristics are controlled by structural and paleogeographic setting, by depositional fabric, and by diagenesis. However, because contours of µH and MH are approximately normal to structure contours but parallel to Smackover thickness contours, it appears that depositional setting (or paleogeography) influenced reservoir heterogeneity more than did structural evolution. The distribution of DP values is not related to pore-facies distribution. Thus we conclude that DP is less useful for regional heterogeneity studies than is MH or µH.

Upper Jurassic Smackover Oil Plays in Alabama, Mississippi and the Florida Panhandle

ABSTRACT Five Smackover (Upper Jurassic, Oxfordian) oil plays can be delineated in the eastern Gulf Coastal Plain. These include the basement ridge play, the regional peripheral fault trend play, the Mississippi Interior Salt basin play, the Mobile graben fault system play, and the Wiggins arch complex play. Plays are recognized by basinal position, relationships to regional structural features, and characteristic petroleum traps. Within two plays, subplays can be distinguished based on oil gravities and reservoir characteristics. Reservoirs are distinguished primarily by depositional setting and diagenetic overprint. The basement ridge play is updip of the regional peripheral fault trend where the Jurassic Louann Salt is thin or absent; structures in this trend formed on pre-Jurassic basement rocks. The basement ridge play is characterized by structural and combination traps. Reservoirs in the Choctaw ridge complex subplay are peritidal, partially to completely dolomitized, oolitic, peloidal, and oncoidal grainstone. Reservoirs of the Conecuh and Pensacola-Decatur ridge complexes subplay are subtidal to supratidal oolitic, oncoidal, intraclastic, and peloidal dolograinstone and dolopackstone, fenestral dolostone, quartz sandstone, and algal doloboundstone. The regional peripheral fault trend play is basinward of the updip limit of the Louann Salt and is typified by salt related structural features. These structural features occur in association with the Pickens, Gilbertown, West Bend, Pollard, and Foshee fault systems and are generally parallel to the basin margin. The regional peripheral fault trend play is exemplified by salt-related structural and combination traps. Reservoirs of the Pickens, Gilbertown, and West Bend fault systems subplay are peritidal, nondolomitic to completely dolomitized, oolitic, oncoidal, and peloidal grainstone. Reservoirs of the Pollard and Foshee fault systems subplay are subtidal to supratidal, partially to completely dolomitized, peloidal grainstone to wackestone, and dolomitized algal boundstone. The Mississippi interior salt basin play is downdip from the Pickens and Gilbertown fault systems and is characterized by structural and combination traps associated with salt tectonism in this basin. Reservoirs are peritidal, nondolomitic to completely dolomitized, oolitic and peloidal grainstone and packstone. The Mobile graben fault system play is located along the eastern limit of the Mississippi interior salt basin and is typified by salt-induced structural and combination traps and Smackover peritidal peloidal and oolitic dolograinstone to dolowackestone and dolostone reservoirs. The Wiggins arch complex play is in a downdip basinal position and is characterized by structural and combination-petroleum traps associated with stratigraphic thinning and salt flow. The traps occur along the flanks of pre-Mesozoic paleohighs associated with this complex. Reservoirs are subtidal to supratidal peloidal, oolitic and oncoidal dolograinstone and dolopackstone, thrombolitic dolostone, and crystalline dolostone.

Sponge-microbial mound facies in Mississippian Tuscumbia Limestone, Walker County, Alabama

A new core from the Black Warrior Basin of Alabama contains a newly discovered mound lithofacies in the Tuscumbia Limestone (Meramecian). The Schlumberger-Alabama Power 1 Plant Gorgas well contains 37.2 m (122 ft) of carbonate rock assigned to the Tuscumbia. This formation overlies the Fort Payne Chert (Osagean), which was penetrated but not cored in this well. The Tuscumbia underlies calcareous shale and limestone of the Chesterian Pride Mountain Formation. Three lithologic units have been defined in the cored part of the Tuscumbia. The basal 2.8 m (9.25 ft) of the core consists of sponge-microbial boundstone (unit 1). This is overlain by 20.6 m (67.6 ft) of mixed carbonate strata dominated by mixed-particle grainstone, which increases in abundance upward (unit 2). Some of the grainstone is brecciated, suggesting exposure and paleokarst development following mound formation. Grainstone is interbedded with thin units of argillaceous cherty peloidal carbonate, sponge-microbial boundstone, and mixed-particle rudstone. Glauconite is common in the basal part of unit 2. Unit 2 is abruptly overlain by 13.8 m (45.2 ft) of bryozoan crinoid grainstone (unit 3) containing breccia beds, low-angle cross-strata, and immature carbonate paleosols. This unit is sharply overlain by the Lewis limestone of the Pride Mountain Formation, which here consists of 0.5 m (1.7 ft) of interbedded fossiliferous shale and mixed-particle packstone. The Lewis limestone is overlain disconformably by fenestrate-bryozoan-rich shale. The basal part of the core records growth of a sponge-microbial mound in relatively shallow water below normal wave base. The mound is not Waulsortian: stromatactis is absent, and matrix material is grainy. The upper contact is sharp, and borings are locally abundant. Abundant authigenic glauconite indicates reducing pore waters. The mound was buried by foreshoal grainstone, much of which is brecciated. Upper Tuscumbia bryozoan-crinoid grainstone formed in a mobile shoal that quickly aggraded to sea level. Although the upper Tuscumbia here is typical of the formation, the shoal buried a mound facies not previously reported from the Tuscumbia. Early diagenesis was dominated by marine cementation, syndepositional alteration, and fracturing. Burial diagenesis was dominated by calcite cementation, dissolution of siliceous spicules, stylolitization, chert formation, and later, emplacement of hydrocarbons and their subsequent transformation to pyrobitumen. Today, original interparticle voids are filled with a mixture of calcite cement, replacive chert, and solid hydrocarbons. Irregular nodules of chert replaced parts of the mound and the overlying heterogeneous unit. Dolomite partially replaced heterolithic strata of unit 2. Patches of once porous rock in units 1 and 2 contain abundant solid hydrocarbons, but porosity (0.7%–5.6%) and permeability (1–78 d) are low. In-place heating led to gas generation and to concomitant in-place solidification of liquid hydrocarbons.

Upward Shoaling Cycles in Smackover Carbonates of Southwest Alabama

ABSTRACT Upper Smackover strata in Alabama commonly consist of one or more upward shoaling cycles ranging from 15 to 50 feet in thickness. These are fourth or fifth order cycles within the third order upper Norphlet to lower Haynesville depositional sequence. Multiple forcing functions (subsidence, salt halokinesis, and autogenic sediment aggradation) and position relative to sea level at the start and end of each cycle generated an array of sedimentary responses. The Brittain No. 1 well illustrates nucleation of an offshore bar. Bar deposits are capped by anhydritic sabkha deposits, gradationally overlain by subtidal lagoonal strata. Varying rates (and directions?) of halokinesis controlled this succession and created as many as five sabkha-capped cycles in the eastern Mississippi interior salt basin. The International Paper Company 20-5 No. 1 well contains three upward shoaling cycles capped by evaporites. Because of the limited aggradational potential of supratidal evaporitic settings subsidence caused immersion, which eventually permitted reactivation of the carbonate factory and formation of the next cycle. The Chatom Unit 20-14 No. 1-04 well contains four different cycles. The lower cycle consists of subtidal lime mudstone, capped by a 5 foot thick paleosol. The paleosol underlies an intraclastic storm deposit followed by a deepening-upward lagoonal succession. A thin ooid grainstone containing exposure surfaces caps the second cycle. The third cycle consists of a deepening upward peloidal carbonate succession capped by closely spaced exposure surfaces. In the upper cycle, peritidal carbonate strata underlie sabkha deposits. The first and third cycles were probably caused by halokinesis; the second and fourth could have been autogenic. Some fourth or fifth order upper Smackover cyclic depositional sequences in southwest Alabama may represent parasequences formed in response to eustatic sea-level change. However, many cycles were caused or influenced by other factors as outlined above.

Desert Environments and Petroleum Geology of the Norphlet Formation, Hatter’s Pond Field, Alabama

Reservoirs of the Norphlet Formation provide good examples of the varied influences of depositional and diagenetic controls on reservoir distribution and quality. Norphlet sandstones are representative of a variety of desert and near-desert depositional settings, and show a considerable range in reservoir quality. The degree to which deep burial of the Norphlet has overprinted and modified the depositional controls of reservoir quality is varied, as seen in this chapter and the two which follow. The Norphlet reservoir in the Hatter’s Pond Field provides an opportunity to compare and contrast the reservoir quality of several desert and near-desert sequences, including wadi, playa dune, and nearshore-marine lithofacies. Reservoir quality varies greatly, and the most productive lithofacies are dune and shoreface sandstones.