Petroleum Source-Rock Potential of Late Paleocene and Early Eocene Wilcox and Claiborne Group Strata Determined by Geophysical Well-Log Analysis: Zapata County, Texas

In the Gulf Coastal Plain of Texas, the Wilcox Group forms a south and eastward prograding deltaic succession that is onlapped by the transgressive marine Reklaw Formation of the Claiborne Group. Few studies of petroleum source-rock potential of this succession exist in relatively distal, downdip, and more deeply buried subsurface sections where mudstone intervals are likely to be more organic rich (less clastic dilution) and oil-prone. The objective of this study is to characterize the variation in organic-carbon richness and sequence stratigraphy of these units by analysis of wireline geophysical logs in a deep test well near the growth-faulted distal margin of the Wilcox depositional shelf.

Lithology and sequence stratigraphy interpreted from wireline logs and the mudlog in the ≈20,000 ft S. G. DeGarcia well (API 42505312370000) of the El Grullo East field, indicate a predominantly mudstone succession with minor sand units present mainly in the upper and lower parts of the Wilcox and prominent transgressive marine mudstone units in the middle Wilcox and lower part of the Claiborne – consistent with previous descriptions. Total organic carbon (TOC) content of selected mudstone intervals in the well was computed using sonic and deep resistivity logs and the ΔLogR technique (Passey and others, 1990) with thermal maturity estimated from wireline logs and regional thermal measurements which suggest that much of the studied section is within the oil window and overpressured. In the absence of thermal maturity measurements from this well, these log-based TOC results are not calibrated. Nevertheless, they show the relative variations in TOC, and these results within the interpreted sequence stratigraphic framework suggest that multiple candidate petroleum source rock intervals (TOC≥2 wt%) are present within at least the upper ≈5,000 ft of the Wilcox Group and the lower ≈2,000 ft of the Claiborne Group at this relatively distal setting.