D. Joe Benson

Upper Jurassic thrombolite reservoir play, northeastern Gulf of Mexico

In the northeastern Gulf of Mexico, Upper Jurassic Smackover inner ramp, shallow-water thrombolite buildups developed on paleotopographic features in the eastern part of the Mississippi Interior Salt basin and in the Manila and Conecuh subbasins. These thrombolites attained a thickness of 58 m (190 ft) and were present in an area of as much as 6.2 km2 (2.4 mi2). Although these buildups have been exploration targets for some 30 yr, new field discoveries continue to be made in this region. Thrombolites were best developed on a hard substrate during a rise in sea level under initial zero to low background sedimentation rates in low-energy and eurytopic paleoenvironments. Extensive microbial growth occurred in response to available accommodation space. The demise of the thrombolites corresponded to changes in the paleoenvironmental conditions associated with an overall regression of the sea. The keys to drilling successful wildcat wells in the thrombolite reservoir play are to (1) use three-dimensional seismic reflection technology to find paleohighs and to determine whether potential thrombolite reservoir facies occur on the crest and/or flanks of these features and are above the oil-water contact; (2) use the characteristics of thrombolite bioherms and reefs as observed in outcrop to develop a three-dimensional geologic model to reconstruct the growth of thrombolite buildups on paleohighs for improved targeting of the preferred dendroidal and chaotic thrombolite reservoir facies; and (3) use the evaporative pumping mechanism instead of the seepage reflux or mixing zone models as a means for assessing potential dolomitization of the thrombolite boundstone.

Improving recovery from mature oil fields producing from carbonate reservoirs: Upper Jurassic Smackover Formation, Womack Hill field (eastern Gulf Coast, U.S.A.)

Reservoir characterization, modeling, and simulation were undertaken to improve production from Womack Hill field (eastern Gulf Coast, United States). This field produces oil from Upper Jurassic Smackover carbonate shoal reservoirs. These reservoirs occur in vertically stacked, heterogeneous depositional and porosity cycles. The cycles consist of lime mudstone and wackestone at the base and ooid grainstone at the top. Porosity has been enhanced through dissolution and dolomitization. Porosity is chiefly interparticle, solution-enlarged interparticle, grain moldic, intercrystalline dolomite, and vuggy pores. Dolostone pore systems and flow units have the highest reservoir potential. Petroleum-trapping mechanisms include a fault trap (footwall uplift with closure to the south against a major west-southeast–trending normal fault) in the western area, a footwall uplift trap associated with a possible southwest-northeast–trending normal fault in the south-central area, and a salt-cored anticline with four-way dip closure in the eastern area. Potential barriers to flow are present as a result of petrophysical differences among and within the cycles, as well as the presence of normal faulting. Reservoir performance analysis and simulation indicate that the unitized western area has less than 1 MMSTB of oil remaining to be recovered, and that the eastern area has 2–3 MMSTB of oil to be recovered. A field-scale reservoir management strategy that includes the drilling of infill wells in the eastern area of the field and perforating existing wells in stratigraphically higher porosity zones in the unitized western area is recommended for sustaining production from the Womack Hill field.

Appleton field case study (eastern Gulf coastal plain): Field development model for Upper Jurassic microbial reef reservoirs associated with paleotopographic basement structures

Appleton oil field, located in Escambia County, Alabama, was discovered in 1983 through the use of two-dimensional seismic reflection data. The field structure is a northwest-southeast–trending paleotopographic ridge comprised of local paleohighs. The field produces from microbial reef boundstones and shoal grainstones and packstones of the Upper Jurassic Smackover Formation. Because Appleton field is approaching abandonment, owing to reduced profitability, an integrated geoscientific study of the field structure and reservoir was undertaken to determine whether drilling additional wells in the field would extend the productive life of the reservoir. The conclusion from the integrated study, which included advanced carbonate reservoir characterization, three-dimensional geologic visualization modeling, seismic forward modeling, porosity distribution analysis, and field production analysis, was that a sidetrack well drilled on the western paleohigh should result in improved oil recovery from the field. The sidetrack well was drilled and penetrated porous Smackover reservoir near the crest of the western paleohigh. The well tested 136 bbl oil/day. (Begin page 1700)

Upper Jurassic (Oxfordian) Smackover carbonate petroleum system characterization and modeling, Mississippi Interior Salt Basin area, northeastern Gulf of Mexico, USA

The Upper Jurassic (Oxfordian) Smackover Formation is a prolific oil and gas reservoir in the northern Gulf of Mexico, including the Mississippi Interior Salt Basin area. The Smackover petroleum system is categorized as a giant petroleum system and ranks fourth among recognized Upper Jurassic petroleum systems. In the Mississippi Interior Salt Basin area, the components of the Smackover petroleum system include pre rift, syn rift and post rift siliciclastic, evaporite, and carbonate underburden and overburden rocks, Smackover subtidal lime mudstone source rocks, uppermost Smackover anhydrite and Buckner Anhydrite Member subaqueous saltern and sabkha seal rocks, and upper Smackover shoal complex and tidal flat complex packstone, grainstone, boundstone and dolostone reservoir rocks. The critical events include the initiation of the generation of crude oil, the commencement of hydrocarbon expulsion, the initiation of hydrocarbon migration, and the entrapment of hydrocarbons during the Early to Late Cretaceous. The critical moment for the Smackover petroleum system is the time of peak hydrocarbon expulsion in the mid to late Early Cretaceous in basin center areas and mid to latest late Cretaceous in areas along the northern basin margin. *** DIRECT SUPPORT *** A00QA034 00003

Diagenetic Controls on Reservoir Development and Quality, Smackover Formation of Southwest Alabama: ABSTRACT

ABSTRACT Smackover reservoirs in southwest Alabama are much more complex than those encountered in the central and western portions of the Gulf Coast basin. This heterogeneity is a product of a complex history of diagenetic modification. The diagenetic history can be divided into five separate stages: (1) Marine Stage - extensive grain micritization and precipitation of minor amounts of fibrous cement, (2) Meteoric Vadose Stage - extensive dissolution of aragonitic grains, (3) Meteoric Phreatic Stage - precipitation of granular and blocky cements, (4) Brine Reflux Stage - extensive dolomitization and sulfate emplacement associated with refluxing of Buckner brines, and (5) Burial Stage - grain to grain compaction of lithologies not stabilized by early cements, stylolitization, precipitation of poikilotopic calcite and dolomite cements, minor dissolution of calcite associated with the introduction into the formation of undersaturated fluids, hydrocarbon migration, and sulfate replacement and cementation. The diagenetic sequence varies dramatically over short distances in southwest Alabama reflecting variation in paleotopography. The products of this complex diagenetic history are heterogeneous reservoirs which consist of one or some combination of primary interparticulate, moldic, dolomitic intercrystalline, and vuggy porosity. Primary interparticulate porosity is less common in southwest Alabama than in the central and western portions of the Gulf Coast basin, largely because of extensive compaction or early cementation. Where present, however, interparticulate reservoirs are characterized by moderate to high porosity and good permeability. Moldic porosity is produced by early, fabric selective dissolution of aragonitic allochems and is associated with areas of subaerial exposure. Moldic reservoirs commonly have high porosity, but low permeability and are generally productive only when permeability is enhanced by other pore types. Dolomitic intercrystalline porosity is common in the Smackover of southwest Alabama. Intercrystalline porosity alone, however, is seldom of reservoir quality and hydrocarbon production from dolomitized lithologies is generally dependent upon the coexistence of moldic or vuggy porosity. Vuggy porosity is the product of late, non-fabric selective dissolution of calcite. Vuggy pores are produced by solution enlargement of earlier formed interparticulate, moldic, or intercrystalline pores. Vuggy reservoirs are by nature composite and are characterized by moderate to high porosity and good permeability.

Geological and Computer Modeling of Upper Jurassic Smackover Reef and Carbonate Shoal Lithofacies, Eastern Gulf Coastal Plain

ABSTRACT Reefs and carbonate shoals have long been known from the Upper Jurassic Smackover Formation in the Gulf Coastal Plain; however, these carbonate lithofacies have unique acoustic properties that make them difficult to define using 3-D seismic reflection technology. In the eastern Gulf Coastal Plain, microbial reef and shoal buildups occur on pre-Jurassic paleotopographic basement features on a carbonate ramp margin. Development of these buildups is a result of the interplay among paleotopography, sea-level changes, and carbonate productivity. Geological and computer modeling indicates that Smackover reef and shoal development is restricted to the flanks of high (emergent) relief structures, while reef and shoal development occurred on the crest and flanks of low-relief structures (submergent). For these submergent features, modeling indicates that carbonate productivity during reef growth was significantly greater than during shoal development. In addition to the initial paleorelief, the rate of sea-level rise, the level of carbonate productivity, and duration of reef growth appear to be critical factors in determining thickness and distribution of the reef lithofacies. Subsidence and compaction are minor factors. Shoal development is greatly influenced by reef distribution, sea-level changes, level of carbonate productivity, and duration of shoal deposition. Subsidence, compaction and sediment redistribution are also factors. Modeling of parameters affecting reef and shoal development associated with pre-Jurassic paleohighs in combination with 3-D seismic reflection studies increases the chances of drilling a successful exploration well.

Petrology and Reservoir Characteristics of Smackover Formation, Hatter's Pond Field--Implications for Smackover Exploration in Southwestern Alabama: ABSTRACT

ABSTRACT Hatters Pond Field in northern Mobile County, Alabama has produced 11 million barrels of condensate and 43 Bcf of gas since its discovery in 1974. Production is from multiple pay zones in the Upper Jurassic Norphlet and Smackover Formations. The trapping mechanism in the field is a highly complex, combination structural and stratigraphic trap involving salt movement in association with normal faulting. The Smackover in the Hatters Pond Field area is enigmatic for the Smackover in Alabama for two principal reasons. One, the Smackover is very thin (less than 200 feet) in comparison to thicknesses to the northwest and southeast. Secondly, the Smackover does not show the characteristic lower Smackover-upper Smackover lithologic subdivision so apparent throughout south Alabama and the Gulf Coast. These unique features are a product of the fields position on the northwest flank of the Wiggins uplift. Smackover deposition was significantly affected by the uplift which maintained the Hatters Pond area as a subaerial high, while lower Smackover carbonates were being deposited in the deeper areas of the Mississippi Interior Salt Basin and Conecuh embayment. It was not until near maximum transgression that the seas covered the Hatters Pond area and deposited shallow-water upper Smackover lithologies. These lithologies were later massively dolomitized by mixing-zone dolomitization during the subsequent Buckner regression. This dolomitization almost completely masked depositional textures, but was largely responsible for the development of reservoir grade porosity in the Hatters Pond area. Six major lithofacies can be identified in the Smackover in the Hatters Pond Field: anhydritic mudstone, skeletal-pelodial packstone/grainstone, oolitic grainstone, microcrystalline dolomite, finely-crystalline dolomite, and coarsely-crystalline dolomite. The microcrystalline dolomite is commonly associated with bedded and nodular anhydrites and is interpreted to represent early replacement in a sebkha environment. Both the finely- and coarsely-crystalline dolomites are secondary in nature and represent replacement of low energy skeletal-peloidal packstones and high energy oolitic grainstones respectively. The majority of the reservoir porosity in the Smackover is late stage vuggy and/or moldic and is facies selective and preferential to the coarsely-crystalline dolomite. This porosity, which commonly ranges from 4 to 22% with permeabilities of 2 to over 100 millidarcies, is a product of mesogenetic leaching related to migration of CO2-charged fluids during the early stages of hydrocarbon maturation. The porosity is facies selective to the coarsely-crystalline dolomite since this lithology possessed the greatest porosity and permeability at the time of migration of the CO2-charged solutions. Evidence suggests the oolitic grainstones, which were the precursors of the coarsely-crystalline dolomites, were deposited as a series of linear bars along the flanks of the Wiggins uplift. If this is the case, and more study is needed to document this definitively, the coarsely-crystalline dolomite should occur in elongate mappable trends. Hydrocarbon exploration in this area and all along the flanks of the Wiggins uplift should involve location and mapping of these trends with the greatest success occuring in areas where the trends are superimposed over structural highs produced by faulting and/or salt diapirism.

Sandstone Hardbottoms along the Western Rim of DeSoto Canyon, Northeast Gulf of Mexico

ABSTRACT A prominent sandstone hardbottom, herein termed the DeSoto Canyon rim feature (DCRF), is present along the western rim of the DeSoto Canyon. The DCRF occurs as a northeast-southwest trending, isobath parallel ridge presently located in approximately 55 m of water. The ridge is over 12 km in length and ranges from 50 to 120 m in width. Relief varies from less than one to over seven meters. The DCRF varies along strike from a broad, low-relief mound to a prominent ridge with a steep, 7-meter high face on the seaward side. The feature displays strong orthogonal jointing. Erosion and undercutting has produced a prominent debris field along the seaward margin. The size of talus blocks indicates lithification extends several meters into the sediment. The DCRF is composed of fine to coarse grained, moderately to well-sorted sublitharenite. Quartz is the dominant framework component with lesser amounts of potassium feldspar and metamorphic and sedimentary rock fragments. Carbonate skeletal grains comprise from 2% to 15% of the lithologies. Terrigenous grains range from subangular to subrounded. The sand is lithified by Mg-calcite cement that makes up from 20% of the lithologies. Cement content is greatest toward the crest of the feature and on the seaward side. Fibrous, bladed, and drusy cements are all present and there is a marked substrate control on cement morphology. Fibrous cements occur as isopachous crusts on carbonate allochems, bladed cements occur as irregular crusts on quartz grains, and drusy cements are present as pore fills and irregular coatings on terrigenous grains. The cements have 18O values that range from +0.1 to +0.8 and 13 C values that range from +0.7 to +2.5. 14C dating of the cements provides average ages from 5,630 ± 70 to 10,560 ± 70 years B.P. Morphology, sedimentary structures, and sediment texture suggest the DCRF is composed of sediments transported by fluvial processes to near the shelf margin during a sea level lowstand and subsequently reworked during the Holocene transgression. Cement precipitation accompanied the Holocene transgression and may have occurred during periods of sea-level stillstand or short-term reversals in sea level.

Diagenesis of the Upper Jurassic Norphlet Formation, Mobile and Baldwin Counties and Offshore Alabama

ABSTRACT The Upper Jurassic Norphlet Formation is an important deep gas reservoir in Mobile and Baldwin Counties and the adjacent Alabama state waters. The reservoir is a well-sorted, fine-grained subarkose to arkose deposited in eolian dune, interdune, wadi, and beach-shoreface environments. Arid depositional conditions and a subsequent marine transgression left an early diagenetic imprint on the sandstone. Near-surface cementation by hematite, quartz, k-feldspar, carbonate, and sulfate minerals occurred concurrently with mechanical compaction. With continued burial, illite and chlorite grain coatings and additional carbonate cements were precipitated. Thermomaturation of organic matter in the overlying Smackover produced carbonic and carboxylic acids and hydrogen sulfide. The introduction of these components into the Norphlet resulted in widespread dissolution of framework grains, grain replacements, and early cements. Pressure solution, saturation of the acid fluids, and invasion of brines from the underlying Louann Salt resulted in the late-stage precipitation of quartz, K-feldspar, anhydrite, and carbonate cements. Porosity in the Norphlet is a mixture of primary intergranular porosity and secondary porosity produced through dissolution of cements, detrital grains, and grain replacements prior to hydrocarbon emplacement. Porosity ranges from zero to 25 percent and averages around 10 percent. Permeabilities range from 0.1 millidarcy to almost 100 millidarcies but are typically less than 1.0 millidarcy. Porosity and permeability vary within the study area as a consequence of geographic differences in diagenetic history and authigenic mineralogy. The best reservoirs occur in northern Baldwin County and in the offshore area.

Random sampling of carbonate mounds: an example from the Upper Ordovician of Alabama

Abstract Paleontological and lithological studies of a carbonate mound provide the necessary data from which characterizations for that mound or locality can be constructed. These data-based characterizations are a convenient mechanism for making qualitative comparisons with other mounds, as has been done in some previous studies that discussed Middle Ordovician (now considered Upper Ordovician) mounds of the Appalachian Basin. Each of these studies had a different focus, including the paleoecology of individual mound localities, issues of ecological zonation, and regional stratigraphical investigation. Quantitative comparisons are precluded among these studies because each mound was sampled using different procedures, resulting in paleontological data sets of dissimilar density and depth. Two mound localities from carbonates of the Upper Ordovician Chickamauga Group (Stones River equivalent) of Jefferson and Blount counties, AL, were chosen for study to investigate the application of random sampling techniques to mound populations in outcrop. One mound from each locality was completely censused to generate population compositional and structural data. The location and higher-level identification of each macrofossil on the surface of these mounds were recorded. These bryozoan-dominated data sets represent the best estimates available concerning the underlying population of mound constructors, dwellers, and occasional “visitors.” Rarefaction analysis was used to predict the number of randomly chosen fossils needed to detect the major taxonomic groups from each of these populations. A computer program (TARGET) was written to validate rarefaction predictions by conducting random sampling experiments using the census data sets. The program prompts for input of three user-defined variables that set the parameters of a sampling experiment and then throws randomly located sampling boxes at the mound data set, recording the results. Statistical analysis of results from these sampling experiments validated the predictions of rarefaction analysis and led us to employ a conservative approach for sampling additional mounds at these localities.