Submarine Cementation Patterns of Holocene Reefs Provide Models for Porosity Development in Ancient Reef Reservoirs: ABSTRACT

Abstract

An understanding of processes of formation and postdepositional alteration of Holocene carbonate buildups can aid the explorationist in locating and predicting reservoir facies in subsurface analogs. In the subsurface, ancient shelf-edge reefs may contain primary porosity that has escaped shallow subsurface cementation. This preserved primary porosity is commonly enhanced later by carbonate dissolution associated with widespread subsurface fluid migration and/or dissolution fronts along permeable stylolite zones. Therefore, given a burial history of continued subsidence, knowledge of early submarine cementation patterns is important for understanding reef facies distribution of late subsurface diagenesis. In reef systems, submarine cementation is controlled by size of sedimentary components, facies energy setting, and reef growth history. Cements are acicular aragonite and dentate Mg-calcite rims, and more commonly thin crusts and geopetal skeletal infills of Mg-calcite peloids. Rapid facies accumulation during reef growth limits submarine cementation to thin rims and incomplete skeletal infilling. Extensive back-reef sediment apron deposits are generally mud free and composed of well-sorted skeletal fragments, that undergo only minor submarine cementation. Reef core (framework) facies contain large amount of in-situ skeletons and increasing mud and peloidal submarine cements within the core matrix. High energy fore-reef facies are extensively cemented by fibrous aragonite druses and d nse peloidal Mg-calcite infill. The best potential reservoir facies are usually back-reef packstone-grainstones, which have greater porosity and permeability because high accumulation rates and moderate energy conditions limited submarine cementation. Following a reef's demise and submergence, submarine cementation of the upper reef surface may form an effective diagenetic End_Page 502------------------------------ seal over the reef that protects internal reef components from additional submarine diagenesis. Consequently, some primary porosity remains intact, that, with continued submergence, may bypass meteoric diagenesis and still remains to become enhanced by late subsurface events. Within core samples of Cretaceous and Miocene reefs, porosity created by late-stage dissolution is facies specific and predominantly moldic and enhanced primary (skeletal and interparticle). Submarine cements occlude some primary porosity in each reef facies. However, back-reef facies result in higher observed porosity because primary permeability allowed greater access for dissolving fluids. Stylolites that form and remain open within reef packstone-grainstone facies act as avenues for fluids that dissolve skeletal grains along narrow adjacent zones within the rock matrix. This late-stage dissolution can produce significant porosity where primary permeability is still preserved. Limited early submarine cementation inhibits burial compaction and acts to preserve porosity. Where stylol tes extend into back-reef mudstone and wackestone facies, a higher percentage of impermeable muds and a limited amount of skeletal grains available for dissolution prevent development of significant porosity. Late-stage subsurface dissolution within reefal buildups, whether by widespread, pervasive fluid migration or as fronts along stylolite zones, is commonly facies-controlled by primary porosity and permeability characteristics. Thus, the distribution and degree of submarine cementation are important to both the early and late development of porosity in reef reservoir facies, even though sometimes for indirect reasons. Figure End_of_Article - Last_Page 503------------