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

Abstract

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.