Tithonian Pimenta Shale, Onshore Mexico: Unconventional Resource Potenial

Abstract

Abstract

In the Gulf Coast of the U.S.A. and Mexico as well as offshore in the Gulf of Mexico Jurassic sediments are key source rocks supplying petroleum to conventional reservoirs. However, these source rocks also have demonstrated large unconventional reserves onshore USA due to retention of considerable portion of petroleum. Any unexpelled petroleum undergoes additional cracking to volatile oil, NGLs, and dry gas with further maturation. The Tithonian Pimienta Shale onshore Mexico shows comparable characteristics to highly successful shale oil and shale gas plays in North America.

The quantity and quality of organic matter in the Pimienta Shale is sufficient for high volumes of petroleum generation and retention. Initial or original TOC and relative hydrogen contents translate into retained petroleum potentials of approximately 70 mmboe/section at 1.00%Roe in the heart of the volatile oil window. Thicknesses are variable but several hundred feet are typical. With further maturation the retained petroleum in the Pimienta is cracked to gas at about 50% efficiency. At dry gas window maturity, this translates into about 200 bcf/section.

At lower thermal maturities the petroleum composition inhibits production from the shale itself. This is due to the petroleum composition which has elevated polar compounds. These compounds are undergoing cracking in the oil window resulting in higher quality oil with increased pressure, i.e., better producibility. At lower thermal maturities, juxtaposed, non-source lithofacies may be a tight oil play possibility where expelled petroleum has enhanced quality due to expulsion fractionation. Expelled oil will have lower polar constituents of petroleum than retained oil resulting in a shift of API gravity of 5-10^oAPI. This, along with the adsorptive affinity of kerogen, explains why the source rock itself, despite containing high volumes of oil, is not necessarily a good shale oil prospect.

Oil saturation indices are variable but often showing oil crossover effect, which is indicative of producible petroleum. Interestingly these intervals are often in the most brittle rocks with high carbonate contents. Of samples showing crossover, the average carbonate content is over 50%. As such carbonates are not only brittle but have lower retention affinities, thereby enhancing producibility.

AAPG Datapages/Search and Discovery Article #90260 © 2016 AAPG/SEG International Conference & Exhibition, Cancun, Mexico, September 6-9, 2016