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Upper Jurassic Cotton Valley Reef Types and Porosity Distribution, East Texas Basin
Detailed petrologic study of the Cotton Valley Limestone and application of Upper Jurassic reef analogues from Europe and North Africa has resulted in the development of an integrated exploration model specific to the East Texas Basin. A variety of reef types exhibiting multiple stages of reef growth occur along the Cotton Valley Reef trend. Cotton Valley reefs grew along the shelf margin from the Late Kimmeridgian to the Kimmeridgian - Tithonian boundary. The reefs are composed of corals, calcareous algae, calcareous and siliceous sponges, and microbolites that formed a progression from shallow to deeper water. The reefs were composed of stacked biostromes or low-relief bioherms. Their pinnacle character or prominent elevation above the sea floor was manifested because reef growth occurred at a time of strongly starved sedimentation within the basin that was associated with arid or semi-arid climatic conditions.
Multi-stage diagenesis has affected porosity, and structural variability along the trend further complicates the geology. Coral-rich reef facies that originally had greater amounts of metastable aragonitic skeletal material were deposited in shallower water depths. These skeletons were more susceptible to leaching and the creation of secondary biomoldic pores. These reefs also had greater potential for subaerial exposure and freshwater diagenesis that resulted in the partial leaching and the creation of matrix microporosity. Those reefs that were located in structurally favorable positions for more frequent or prolonged periods of exposure appear to have thicker and more numerous porous zones.
Substantial macroporosity was occluded by late burial cementation; however, early hydrocarbon migration helped to preserve porosity.
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