Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
American Association of Petroleum Geologists Annual Meeting, Dallas, April 18-21, 2004
Petrophysical Evaluation of Miocene-Pliocene Gas Reservoirs: Veracruz and Macuspana Basins, Mexico
Shinichi Sakurai, William Ambrose, D. C. Jennette, Mark H Holtz, Shirley Dutton, K. Fouad, T. F. Wawrzyniec, Dallas Dunlap, Edgar H. Guevara, Francisco M. Grimaldo-Suarez, Leonardo Enrique Aguilera-Gomez, and Jose A. Rodriguez-Larios
PEMEX Exploración y Producción and the Bureau of Economic Geology conducted a basin analysis and play characterization study of Pliocene and Miocene gas plays in the Veracruz and Macuspana Basins in southern Mexico. An integrated petrophysical analysis provided valuable inputs for stratigraphic studies and for understanding the gas-producing reservoirs.
We analyzed more than 350 wells from these two basins, which were drilled mostly in the 1960s and 1970s. Substantial time was spent on database building and quality control. Petrophysical studies consisted of lithological determination, construction of a log model, and petrophysical assessments. Understanding the lithology distribution was one of the keys for this study. Core description, cutting descriptions and final well reports were used to build and calibrate lithology models. Sand, shale, conglomerate, igneous rock, and limestone are estimated. This lithological information was useful for stratigraphic analysis and tying with seismic data.
Petrophysical assessments were helped by special core analysis. Capillary pressure results were compared against lithological information, including thin-section pictures. Unique porosity and saturation relationships were observed for each lithology, and specific parameters to determine net gas pays for different lithologies were chosen.
Geopressured zones are present, which are recognized by slowing acoustic velocity. The distribution of the geopressured zone was mapped. Results indicate a complex geopressure system, present in the several stratigraphic intervals and also variable depth intervals. Heavy drilling-mud is commonly used, and drilling difficulties are found in these geopressured intervals. 3-D seismic data will help recognition of these pressured intervals.