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Importance of Micropore Networks in Shallow-Water Unconventional Carbonate Reservoirs of the Cretaceous Stuart City Trend


Recognizing the presence of microporosity in carbonate reservoirs is often a critical step in the characterization process. High microporosity/total porosity ratios adversely affect production rates and create zones of high water saturation and need to be fully appreciated. Microporosity is commonly associated with deep-water chalks and mudrocks; however, micropore networks can be important contributors to porosity and permeability in shallow-water carbonates as well. The Albian (Cretaceous) Word Field in Lavaca County, Texas, is a 600 BCF gas field that produces from microporous stacked subtidal cycles deposited in a lagoon behind the Stuart City Reef margin. This study aims to delineate spatial trends in micropore distribution in Word Field by integrating data from four cores and sixty thin sections with six-inch to one foot spaced core plug porosity and permeability values. Methodology includes picking depositional facies and building a sequence stratigraphic framework from 1D and 2D stacking pattern analysis. Investigation of depositional and diagenetic textures in thin section suggests that microporosity is not facies selective at Word Field, instead forming after high-magnesium calcite allochems that occur throughout the shallow-water platform interior setting. Key microporous grains include the cortices of oncoids and other microbially coated grains, including encrusting Lithocodium. Minor pore types include moldic porosity after fossils and trace interparticle porosity in lower energy peloidal facies. Comparisons of petrophysical data with the sequence stratigraphic framework at several scales suggests that relationship between microporosity and permeability is most predictable at the system tract scale within high-frequency sequences. Correlation along depositional strike among the cored intervals shows a moderate degree of heterogeneity. Additionally, lateral changes in accommodation at the system tract and sequence scales affect the thickness and frequency of different facies packages. Fully understanding the microporosity distribution involves integrating observations at the high-frequency cycle level with larger scale changes in stratigraphy throughout the field. The overall aim of the study is to improve our understanding of microporosity in shallow water carbonate and to add to our knowledge of the Stuart City trend and other Cretaceous platforms.