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Matagorda Island 623 field is a large gas accumulation in overpressured, lower Miocene Siphonina davisi deltaic sandstones. Production is from fine, moderately well sorted sublitharenites deposited in distributary-mouth bars and channels. Reservoir depth is 10,000-14,000 ft (3,050-4,275 m) and bottom-hole temperatures approach 275°F (135°C). Pay-sand porosities range from 15 to 35% and permeabilities range from 10 to 3,000 md.
Between 60 and 85% of the porosity is primary intergranular. Porosity preservation is dependent upon the following: (1) early formation of chlorite grain coats, (2) a stable mineralogic framework, (3) overpressuring, and (4) entry of gas into the reservoir. Chlorite coats form as much as 8% of the rock and originated from a chemical decomposition of volcanic rock fragments (VRF). These coats inhibited quartz cementation and retarded compaction related to pressure solution.
Secondary porosity originated mainly from feldspar and VRF dissolution. Although calcite cement is locally common, evidence is lacking that calcite leaching formed significant porosity. Evidence against large-scale carbonate dissolution includes the following: (1) unaltered carbonate lithoclasts, (2) reworked Cretaceous foraminifers, (3) preservation of delicate chlorite crystal morphology, and (4) the absence of pitted or serrated surfaces on quartz overgrowths, which formed prior to calcite cementation.
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