Search

With the recent discovery of hydrocarbons in microbial carbonate reservoir facies along the South Atlantic margins, industry is keen to further the understanding of the origin and development of nonmarine (lacustrine) to marine microbial carbonates, the nature of the depositional and diagenetic characteristics of microbialite and associated facies, and the sedimentary and petrophysical properties of microbial carbonate petroleum reservoirs. In this regard, the authors of AAPG Getting Started Series 19, Microbial Carbonate Reservoirs , Mancini et al. (2010), found that a vast literature on microbes and their functions in geological processes and products ranging from biogeochemistry to geomicrobiology, sedimentary petrology, and stratigraphy was available; however, only a small part of this literature deals with microbial carbonates as petroleum reservoirs. Further, these authors reported that sparse available literature exists on microbial carbonate reservoirs formed in lacustrine and continental settings and on carbonate reservoirs of abiotic origin in general. To help address these shortcomings, an AAPG Hedberg Research Conference was proposed to focus on microbial carbonate reservoir characterization with an emphasis on depositional settings of microbial and abiotic carbonate reservoirs, especially in lacustrine settings associated with rift basins. The proposal was accepted August 21, 2010, and the Hedberg Conference was held in Houston, Texas, last June 3–8, 2012, with 85 attendees from 10 countries. The conference was dedicated to Wayne M. Ahr, who passed away in November 2011. He was one of the conference organizers, one of the intended field-trip leaders, and a friend to many at the conference. The following overview of microbial carbonates is modified from sections 1 and 2 of AAPG Getting Started Series 19, Microbial Carbonate Reservoirs (Mancini et al., 2010). Microbes are defined as microorganisms visible only under a microscope. Some examples are bacteria, fungi, molds, algae, and protozoa. Unlike the older classification of living organisms …

Outcrop studies of sedimentary and stratigraphic features of microbial buildups are important in developing conceptual models for exploration of potential microbialite reservoirs. This study examines the occurrence and distribution of Lower Ordovician microbial fabrics in central Missouri and Kansas. The units studied include the Roubidoux, Jefferson City, and Cotter Formations of the Arbuckle Group. To determine the depositional environments suitable for microbialite development, micro-, meso-, and macroscale features were examined, and their stratigraphic relationships were established to the surrounding lithofacies. Microscopic-scale analysis reveals heavy dolomitization of strata; however, remnant depositional features provide vital clues about the microbialite development. The occurrence of microbial structures is placed within a cycle stratigraphic framework. The microbialites occurred within cyclical shallow-marine deposits and are grouped into three cycle types. Type 1 deposits include subtidal mudstones to packstones that grade into intertidal stromatolites. Type 2 deposits are composed of subtidal mudstones and packstones with thrombolite fabrics that grade into supratidal facies. Type 3 deposits include intertidal or supratidal facies that grade into supratidal facies.

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.

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.

The “lower” Ellis Group (M. Jurassic) of northern Wyoming and southern Montana affords an excellent opportunity to examine the influence of tectonics, sea-level change, and incipient topography on facies dynamics and the evolution of mixed sediment ramp deposits. The Sawtooth, Piper, and Gypsum Spring formations (Bajocian to Callovian) represent sedimentation along the forebulge of a retro-arc foreland basin. The “lower” Ellis Group records deposition during two transgressive–regressive cycles, (1) a Bajocian-age cycle dominated by evaporites and red shales, and (2) a Bathonian-age cycle characterized by carbonates, evaporites, and red shales. These cycles are capped by a Callovian-age cycle distinguished by carbonates and red shales that is represented by the “lower” Sundance and Rierdon Formations. Transgressive episodes favored intensified chemical sediment production resulting in thick units deposited in subtidal to peritidal environments. Regressive periods are characterized by supratidal redbed progradation and subsequent shallowing–upward cycles. The depositional cycles in the lower Ellis Group developed due the interplay between sea-level change and tectonic subsidence related to the evolution of a retro-arc foreland basin. Differential subsidence before, during, and after deposition created paleohighs that locally influenced accommodation space and, thereby, complicated depositional and erosional patterns. This paper provides a regional framework for further analysis of the depositional history of the lower Ellis Group by addressing the stratigraphic relationships between the Sawtooth, Piper, and Gypsum Spring Formations.

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.

A three-dimensional (3D) numerical stratigraphic model has been built to simulate the development of microbial reefs on a Jurassic carbonate platform using fuzzy logic modeling methods. The flexibility of the model allows for the examination of a large number of variables. The relative importance of local environmental conditions and global changes on the frequency of reef development are examined relative to the temporal and spatial constraints from Upper Jurassic (Oxfordian) Smackover reef datasets from two Alabama oil fields. The fuzzy model simulates the deposition of reefs and carbonate facies by integrating local and global variables. Local-scale factors include basement relief, sea-level change, climate, latitude, water energy, water depth, background sedimentation rate, and substrate conditions. Regional and global-scale changes include relative sea-level change, climate, and latitude. The model confirms preexisting theories that background sedimentation rate, water energy, substrate, relative sea-level change, and climate influenced the development of Jurassic reefs. Additionally, the model determines the degree of control each variable has on 'IF,' 'WHERE,' and 'WHEN' reefs develop. The variables that control 'IF' reefs develop are the global parameters of climate and amplitude of sea-level change. The model specifies that the variables that influence the lateral distribution of reefs (or 'WHERE' reefs occur) are substrate, water energy, and background sedimentation rate. The model also indicates a strong correlation between 'WHEN' reefs develop and the rate of sea-level rise. In all simulations, reefs initially develop in conjunction with the greatest rate of sea-level rise (i.e., the inflection point on the sea-level curve) and reach peak development before the time of deepest water.

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.

Symbolism, allegory, and metaphor pervade Athanasius Kircher’s (1602–1680) Mundus Subterraneus (The Subterranean World). Elements from the communicative theory of semiotics are useful for exploring Mundus Subterraneus and for illuminating the modern reactions to his works. Kircher used Hermetic and Neoplatonic philos ophies as a bridge between medieval thought systems and the growing empirical movement of the Scientifi c Revolution. In Kircher’s studies, no event was taken in isolation, and his examination of the earth rested with Plato’s philosophy that the world was created by God as a manifestation of his own perfection. From a modern semiotic viewpoint, Kircher used indexical and iconic signs to combine rational and empirical techniques that sustained his holistic view of the cosmos. In the modern ideal formulation of scientifi c observation and inquiry, indexical signs are acceptable authoritative causal links between observation and interpretation. For Kircher, both indexical and iconic signs were legitimate articles to collect and employ because they were all manifestations of the Divine Mind. Iconic signs could be religious images or conceptual ideas that Kircher projects onto the workings of the earth.

The principal research effort for Year 1 of the project is T-R cycle characterization and modeling. The research focus for the first nine (9) months of Year 1 is on outcrop study, well log analysis, seismic interpretation and data integration and for the remainder of the year the emphasis is on T-R cycle model development.

The Smackover Formation, a major hydrocarbon-producing horizon in the Mississippi Interior Salt Basin (MISB), conformably overlies the Norphlet Formation and is conformably overlain by the Buckner Anhydrite Member of the Haynesville Formation. The Norphlet-Smackover contact can be either gradational or abrupt. The thickness and lithofacies distribution of the Smackover Formation were controlled by the configuration of incipient paleotopography. The Smackover Formation has been subdivided into three informal members, referred to as the lower, middle and upper members.

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.

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)

A new computer model predicts depositional facies distribution and productivity rates on a 3D carbonate platform using continuous set theory (fuzzy logic), which accepts complex, linguistic data with large uncertainties. Earlier deterministic models have become valuable tools for stratigraphic interpretation, but have a difficult time with linguistic concepts whose parameters are difficult to define. For example, deterministic models estimate "depositional facies" based solely on water depth. The term "depositional facies" involves many more complex, nonlinear processes including water energy, circulation, climate, and distance from siliciclastic input. The current model integrates water depth with such complex variables. The user approximates parameters for each of the variables using linguistic terminology. The model produces depositional facies including sabkha, tidal flat, reef, shoal, and subtidal. Carbonate productivity is controlled by many complex processes such as water depth, temperature, latitude, turbidity, climate, and water energy. Deterministic models use either simple water-depth dependent functions or apply a constant rate to model productivity within the shallow water zone. Both deterministic methods unrealistically apply the same productivity rates across a model's extent. The current model integrates water depth with interrelated variables allowing variation in productivity depending on processes occurring at specific locations. Development of this model makes it possible to incorporate linguistic and imprecise geologic data into stratigraphic simulations and this allows visualization of the response of carbonate systems to interactions of complex processes. This also allows greater potential for accurately predicting carbonate reservoir distribution.