Abstract
The Cretaceous Eagle Ford of South Texas is a major unconventional play. Age equivalent rocks are present in the adjacent Burgos Basin, Mexico along with other unconventional targets in the Jurassic. The objective of this study was to map areas of unconventional potential from basinwide maturity predictions provided by 3D modeling. This study has identified oil, wet gas and dry gas areas of interest for the Cretaceous and Jurassic targets. These areas of interest can then be used to focus followup studies by companies or institutions evaluating joint ventures and/or lease sale blocks in the basin.
The 3D model for the Burgos Basin was made using publicly available information. Regional structure maps were made by integrating published structure maps and cross sections. Structure maps, temperature gradients from well logs and a tertiary erosion map were the key inputs used to model maturity. The Cretaceous Agua Nueva and the Jurassic La Casita/Pimienta Formations were the primary zones of interest. Rock maturity data was available for one Cretaceous and one Jurassic well. The model was also verified by comparing to Cretaceous and Jurassic unconventional well results.
Structural strike of the Eagle Ford in south Texas is southwest to northeast. Near the border structural strike abruptly changes to nearly north - south. In the Burgos Basin, the Mesozoic section dips eastward toward the Gulf of Mexico due to over 30,000 feet of Tertiary sand and shale deposition. Faulting in the Tertiary section generally soles out above the Mesozoic, so the Mesozoic is mostly tectonically undisturbed which is favorable for unconventional targets.
The prospective area for the Cretaceous and Jurassic is essentially coincident and is over 40 miles wide and 300 miles long. The prospective area was defined according to depth and modeled vitrinite reflectance equivalence (VRE). Measured depths of 5,000 to 15,000 feet and VRE greater than 0.8 were used. The rationale was that shallower than 5,000 feet would have low pressure and temperature and greater than 15,000 feet would have too high a well cost for horizontal wells. The oil prospective area is from 0.8 to 1.1 VRE, wet gas from 1.1 to 1.7 VRE and dry gas over 1.7 VRE. Oil spacing was assumed to be 100 acres and gas spacing 200 acres. Total recoverable resources are estimated at approximately 27 BBOE of which 15% are liquids (oil and condensate) and 85% are gas.
