William C. Parcell

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.

Sequence stratigraphic controls on the development of microbial fabrics and growth forms-implications for reservoir quality distribution in the Upper Jurassic (Oxfordian) Smackover Formation, eastern Gulf Coast, USA

This paper presents a model that integrates sequence stratigraphic concepts with a fabric and growth form classification of Smackover microbial buildups to aid in understanding the distribution of reservoir quality in the updip basement ridge play of southwest Alabama. Microbial growth forms and fabrics, early diagenetic processes, and resulting reservoir quality are all ultimately controlled by the rate of relative sealevel change, position of sea level with respect to exposed Paleozoic basement, and position in an inner ramp setting. The microbial classification divides fabrics and growth forms into five “types,” which developed in response to changes in water energy, sedimentation rate, and substrate. Layered thrombolite with characteristic mm/cm-scale crypts characterize Type I buildups. Reticulate and “chaotic” thrombolite comprise Type II buildups. In the updip basement ridge play, the Smackover sea did not flood the Paleozoic basement until deposition of the sediments associated with the late transgressive systems tract. Layered and reticulate thrombolite buildups (Types I and II) grew directly on Paleozoic basement and formed in reponse to late transgressive systems tract catch-up conditions when sedimentation rates were low and water energies were moderate to high. Both Type I and II buildups occur on low and high relief basement structures. Type III buildups are characterized by dendroidal thrombolites. On low relief basement structures, dendroidal thrombolite buildups (Type III) typically overlie Type I and II buildups. Type III buildups are absent on high relief structures. Dendritic thrombolites grew in early highstand systems tract keep-up conditions when sedimentation rates were slightly elevated and water energy low. These conditions occurred on the tops of low-relief basement structures associated with early highstand systems tract deposition. Type IV microbialite are composed of isolated stromatolitic crusts that acted as binders to Type V oncoidal packstone/grainstones that grew on soft to firm substrates in high-energy conditions. Abundant in the late highstand systems tract deposits, Type IV (isolated crusts) and V (oncoid) microbialite are found in upper Smackover shoal, lagoon, and tidal flat facies.Classification of microbial types is significant to hydrocarbon exploration and production in southwest Alabama. Types I, II, and III buildups are the best fabrics for productive reservoirs. Of these, Type III buildups are the highest quality reservoir rocks. Dolomitized reticulate and dendritic fabrics result in well-connected intercrystalline and vuggy porosity. Type IV and V microbialite are poor reservoir rocks because Type IV forms are often isolated, and the moldic porosity associated with Type V oncoids are typically not well connected.

Sequence-stratigraphic analysis of Jurassic and Cretaceous strata and petroleum exploration in the central and eastern Gulf coastal plain, United States

The formulation of an integrated sequence-stratigraphic and biostratigraphic framework is fundamental in the design of an effective strategy for petroleum exploration in a sedimentary basin. For the interior salt basins of the Gulf coastal plain of the United States that are filled primarily with Mesozoic postrift nonmarine to marine siliciclastic and carbonate deposits, a sequence-stratigraphic approach using transgressive-regressive (T-R) sequences and integrated with biostratigraphic information has utility as a method for establishing such a framework. The sequence stratigraphy established for Upper Jurassic and Cretaceous strata is used to categorize petroleum reservoirs in the central and eastern Gulf coastal plain. Transgressive aggrading eolian, fluvial, and coastal sandstone facies of the T-R sequences include highly productive hydrocarbon reservoirs in the eastern Gulf coastal plain. Productive reservoirs in the central and eastern Gulf coastal plain include regressive infilling fluvial to nearshore marine sandstone facies, and nearshore marine, shelf, ramp, and reef carbonate facies. Transgressive backstepping nearshore marine facies include highly productive reservoirs in the central Gulf coastal plain. These transgressive and regressive facies are recognized by their wireline log patterns and seismic reflection configurations. Knowledge of the diagnostic wireline log signatures and seismic reflection characteristics assists in the detection of exploration targets.

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 updip stratigraphic trap and associated Smackover microbial and nearshore carbonate facies, eastern Gulf coastal plain

The development of Little Cedar Creek field in the eastern Gulf coastal plain of the United States has shown that the current exploration strategy used to find hydrocarbon-productive microbial and high-energy, nearshore carbonate facies in the Upper Jurassic Smackover Formation requires refinement to increase the probability of identifying and delineating these potential reservoir facies. In this field, the petroleum trap is a stratigraphic trap characterized by microbial boundstone and packstone and nearshore grainstone and packstone reservoirs that are underlain and overlain by lime mudstone and dolomudstone to wackestone and that grade into lime mudstone and dolomudstone near the depositional updip limit of the Smackover Formation. Reservoir rocks trend from southwest to northeast in the field area. The grainstone and packstone reservoir is thickest in the central part of the field. The boundstone reservoir is thickest in local buildups that are composed of thrombolites in the southern part of the field and is absent along the northern margin. These reservoir facies are interpreted to have accumulated in water depths of approximately 3 m (10 ft) and in 5 km (3 mi) of the paleoshoreline. In contrast to most other thrombolites identified in the Gulf coastal plain, these buildups did not grow directly on paleohighs associated with Paleozoic crystalline rocks. The characterization and modeling of the petroleum trap and reservoirs at Little Cedar Creek field provide new information for use in the formulation of strategies for exploration of other Upper Jurassic hydrocarbon productive microbial and related facies associated with stratigraphic traps in the Gulf coastal plain.

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

Outcrop Analogs for Reservoir Characterization and Modeling of Smackover Microbial Reefs in the Northeastern Gulf of Mexico Area

ABSTRACT Upper Jurassic (Oxfordian) Smackover microbial reef buildups in the northeastern Gulf of Mexico consist primarily of shallow-water thrombolites that develop on hardgrounds or rockgrounds associated with Paleozoic basement paleohighs. These microbolites are typically associated with late transgressive to early highstand systems tract deposits and range from 10 ft (3 m) to 150 ft (45 m) in thickness. Because the Smackover strata are not exposed at the surface in this region, determination of the geometries and extent of these thrombolitic buildups, which are critical to devising hydrocarbon development strategies for these reservoirs, is difficult. Upper Jurassic (Oxfordian and Kimmeridgian) microbial buildups are exposed in Western Europe where they are also associated with late transgressive to highstand systems tract deposits. In France, these microbial reefs have been observed in outcrop to consist of layered to dendroidal thrombolitic buildups that generally do not exceed 10 ft (3 m) in thickness. In Portugal, microbial reefs consist primarily of thrombolitic buildups that attain thicknesses of 100 ft (30 m). These buildups are associated with sediment starvation surfaces and hardgrounds and occur in early highstand systems tract deposits. The use of Upper Jurassic microbial reef outcrops in Europe to characterize and model the Smackover thrombolitic reefs in the subsurface of the northeastern Gulf of Mexico greatly facilitates the design of hydrocarbon development strategies for delineating Smackover microbial reefs through 3-D geologic and seismic modeling.

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.

Evaluating and Communicating Geologic Reasoning with Semiotics and Certainty Estimation.

Cognitive and conceptual uncertainties are critical elements in geology from the earliest data collection stage to concluding interpretations. How a geologist conceptually weighs the importance of various data greatly influences final interpretations. In order for the process of data selection and interpretation to be transparent and repeatable, field methods and analyses should be able to communicate these cognitive processes, yet such uncertainty is difficult to characterize and estimate with standard statistical methods based on frequency probability. Semiotics and expert systems are used to frame discussion of the methods of stratigraphic reasoning and develop a field-based method to communicate cognitive and conceptual uncertainty from data collection to final interpretation. In semiotics, signs are the meanings an observer gives to an object to reach an interpretation. Uncertainty in knowledge is derived from problems in recognizing objects and judgment of inferred relevance to a larger interpretation. A final measure of geologic interpretation certainty (IC) can be derived from the product of the confidence factor (CF) of a measurement and the relevance factor (RF) of that object to an interpretation. Communicating levels of interpretation certainty, relevance, and significance allows geologic investigations to be more transparent to subsequent geoscientists, non-geologists, and even the original investigator.

Stratigraphic Architecture of Upper Jurassic (Oxfordian) Reefs in the Northeastern Gulf Coast, U.S. and the Eastern Paris Basin, France

ABSTRACT The Late Jurassic was a major period of reef expansion. Many Jurassic reefs are potential hydrocarbon reservoirs, but their stratigraphic distribution has been difficult to predict. Examination of the stratigraphic architecture of Oxfordian reefs in the northeast U.S. Gulf Coast and the eastern Paris Basin, France affords a comparison of mixed coral-algal sponge reefs versus microbial-dominated reefs. The Smackover Formation in the northeastern Gulf Coast contains microbial-dominated reefs. The Smackover represents deposition during a major transgression and reefs occur within late transgressive system tract (TST) deposits. Reefs of mixed coral, algae, and sponges characterize the Oxfordian of the eastern Paris Basin. In the Ardennes-Lorraine, reefs appear after the maximum flooding event of a stratigraphic sequence and are maintained through the highstand systems tract (HST) deposition and into the overlying stratigraphic sequence. In Burgundy, reefs occur in association with the maximum flooding event of the upper stratigraphic sequence. Dominated by platy microsolenids, reef growth begins in the TST. This interval is traditionally called the lower reef complex. More diverse reef growth (the upper reef complex) continues above the maximum flooding event through the early HST. Reef growth ceases by deposition of the late HST with the introduction of grainstones and other high-energy lithologies. All reefs in this examination, regardless of biota, are associated with maximum flooding events. The sequence stratigraphy of the Oxfordian reefs in France and the Gulf Coast provides a framework for studying Jurassic reefs of both mixed and microbial-dominated assemblages.