--> Abstract: Geochemistry of the Matagorda Island 519 Field, Offshore Texas, by B. Klein, L. Hathon, and C. Brewster; #90932 (1998).

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Abstract: Geochemistry of the Matagorda Island 519 Field, Offshore Texas

KLEIN, BOB
Amoco, Houston, TX;
LORI HATHON
Shell E&P, Houston, TX;
CHARLES BREWSTER
Texaco, New Orleans, LA

Offstructure production of hydrocarbons from Lower Miocene sandstones occurs at the geopressured Matagorda Island 519 field. Sandstone porosities in the 20-25 % range are the result of dissolution of an early diagenetic phase of calcite cement. Intergranular volumes of 25-35 %, burial history profiles and 5180 values of calcite cement suggest that sands were cemented relatively early at depths of 1,000 to 8,000 ft. Diagenetic zonation within the sandstone bed is apparent. Polycrystalline quartz and pyrite cement occur updip whereas patchy ankerite and Fe-calcite cements are found downdip in the reservoir facies. The updip portion of the sandstone bed is located proximal to a regional growth fault that may have acted as a conduit for basinal fluids. We hypothesize that basinal fluids were episodically focused into the sandstone bed, and that distance from the growth fault controls the nature and extent of diagenetic alteration.

Rock samples from reservoir intervals of field cores and wellhead gases were analyzed isotopically to test that hypothesis. {87/86}Sr values of calcite cement range from 0.70805 to 0.71011. These values eliminate coeval Miocene seawater as the only source of calcite. Furthermore, the most radiogenic strontium values are observed updip and proximal to the regional growth fault. Moreover, sulfur isotope values of H2S (1.9 to 3.9 per mil, CDT) collected at the wellhead are similar to d34S of gypsum of the deeply buried Eagle Mills Formation. Such d34S values of H2S could result from thermochemical sulfate reduction of the Eagle Mills evaporite. Results of geochemical analyses are consistent with a model in which the deeply-seated growth fault serves as a conduit for hydrocarbons and diagenetic fluids.

AAPG Search and Discovery Article #90932©1998 GCAGS/GCS-SEPM Meeting, Corpus Christi, Texas