Drilling and production onshore and offshore Mexico have substantiated U. Jurassic source rocks as effective source rocks for conventional and now unconventional petroleum resources. The U. Jurassic section in the Tampico-Misantla basin has excellent source rock characteristics for oil generation based on good original TOC values and hydrogen indices indicative of oil prone marine organic matter. From a mineralogical viewpoint, these shales (or mudstones) are generally very high in carbonate content that is conducive to unconventional production potential.
Data from various source rock oil reservoir systems show that the overall petroleum composition is significantly different from juxtaposed, organic-lean, non-source rock intervals as well as produced petroleum at the wellhead or separator. Thus, it is necessary to understand the in situ petroleum composition of the source rock if that is the completion objective. Petroleum composition affects physicochemical properties such as API gravity, viscosity, and gas-to-oil ratio (GOR). For example, the amount of the polar compounds (NSOs) is inversely proportional to API gravity and saturated hydrocarbon content. It then is important to discriminate the oil type and related phase as black oil, volatile oil, condensate-NGL, and dry gas by correlative and direct indications of such composition.
At higher thermal maturities, oil quality increases rapidly as the non-hydrocarbon constituents of petroleum are cracked to hydrocarbons and refractory carbonaceous residues. There is an exponential increase in GOR in the volatile oil to the earliest gas window, albeit appearing linear in the black oil window. The key is to relate the physicochemical properties of petroleum to correlative tools, i.e., thermal maturity measurements, and direct indicators of sub-surface oil properties and phase.
A combination of visual and chemical measurements, when available, are used to risk thermal maturity assessments including vitrinite and solid hydrocarbon reflectance, Tmax, hydrogen index, gas composition, gas carbon isotopes, and aromatic hydrocarbons. These values are integrated into an interpretive thermal maturity and related to surface and subsurface API gravity and GOR values. Reasonable thermal maturity interpretation allows restoration of the petroleum generation potential for any given sample analyzed by determining the level of kerogen conversion and also taking into consideration secondary cracking. This permits computation of restored TOC, hydrogen indices, and pyrolysis (S2) yields. From these results, the total generation potential can be estimated along with expelled and retained petroleum contents. Comparing these reverse model results with forward model results using restored total oil (S1total) allow a check and a comparative estimate of expulsion as otherwise estimated. These predictions are further checked by slope factor analysis allowing PVT-like restoration of oil and gas yields.