Estimate of Lithology and Net Gas Sand from Wireline Logs: Veracruz and Macuspana Basins, Mexico

By

SAKURAI, SHINICHI

Bureau of Economic Geology, John A. and Katherine G. Jackson School of Geosciences, The University of Texas at Austin, Austin, TX,

GRIMALDO-SUAREZ, FRANCISCO M., AGUILERA-GOMEZ, LEONARDO E., and

RODRIGUEZ-LARIOS, JOSE A.

PEMEX Exploración y Producción, Veracruz and Macuspana, Mexico

PEMEX drilled many wells in the Veracruz and Macuspana Basins of southern Mexico from the 1950’s through the 1970’s looking for large oil fields. As a result, several gas and condensate fields in Pliocene and Miocene intervals were discovered. In 2001, the Bureau of Economic Geology and PEMEX completed basin analysis and play characterization, including petrophysical analysis. The type of data acquired was limited, no wireline pressure data are available, and only a few wells have core description and core analysis results. Our analysis therefore relied on older wireline logs and information from mud logs.

Lithology distribution was a focus of the study because gas-producing reservoirs are porous sandstones (up to 35 p.u.) in fluvial, deltaic, and basin/slope systems, as well as low porosity conglomerates in channels within slope and fan systems. In addition to core description, cutting descriptions from mud logs and final well reports were used to build lithology models from wireline logs. Sand, shale, conglomerate, and igneous rock were also estimated. These lithological estimations were useful for stratigraphic correlation with wireline logs and seismic data.

Special core-analysis tests are available for several core samples with varying that have different lithologies. Distribution of pore systems for each rock type was reviewed by using capillary pressure results, along with detailed petrographic description. To determine net-gas intervals, unique relationships between porosity and saturation were established, and specific cutoff parameters were chosen for each lithology. Results were compiled and used in basin analysis and gas-play characterization.