Fracturing of low-permeability source rocks is induced by pore-pressure changes caused by the conversion of organic matter to less dense fluids (oil and gas); these fractures increase the permeability and provide pathways for hydrocarbon migration. An equation for the pressure change is derived using four major assumptions. (1) The permeability of the source rock is negligibly small (0.01 µd; 10-20 m2) so that the pore-pressure buildup by the conversion is much faster than its dissipation by pore-fluid flow. (2) The stress state is isotropic so that horizontal and vertical stresses are equal. The source rock fails when the pore pressure equals the overburden pressure. (3) The properties of the rock, organic matter, and fluids remain constant during oil generation. This assumption is valid when the change in depth (i.e., pressure and temperature) is small. (4) Only two reaction rates are required for the conversions, a low-temperature reaction rate for the kerogen/oil conversion (E approx. = 24 kcal/mol, A approx. = 1014/m.y.) and a high-temperature reaction rate for oil/gas conversion (E approx. = 52 kcal/mol, A approx. = 5.5 ´ 1026/m.y.). The equations for generation rate and pressure change are applied to the Austin source rock by adjusting the several variables to fit geochemical data, core saturations, and observed levels of oil and gas production. This application demonstrates that the equations are easily applied in calculating depths of primary migration for low-permeability source rocks.

Primary Migration by Oil-Generation Microfracturing in Low-Permeability Source Rocks: Application to the Austin Chalk, Texas

As part of an assessment of undiscovered hydrocarbon resources in the northern Gulf of Mexico onshore Mesozoic section, the U.S. Geological Survey (USGS) evaluated the Lower Cretaceous Pearsall Formation of the Maverick Basin, south Texas, as a potential shale gas resource. Wireline logs were used to determine the stratigraphic distribution of the Pearsall Formation and to select available core and cuttings samples for analytical investigation. Samples used for this study spanned updip to downdip environments in the Maverick Basin, including several from the current shale gas-producing area of the Pearsall Formation. The term shale does not adequately describe any of the Pearsall samples evaluated for this study, which included argillaceous lime wackestones from more proximal marine depositional environments in Maverick County and argillaceous lime mudstones from the distal Lower Cretaceous shelf edge in western Bee County. Most facies in the Pearsall Formation were deposited in oxygenated environments as evidenced by the presence of biota preserved as shell fragments and the near absence of sediment laminae, which is probably caused by bioturbation. Organic material is poorly preserved and primarily consists of type III kerogen (terrestrial) and type IV kerogen (inert solid bitumen), with a minor contribution from type II kerogen (marine) based on petrographic analysis and pyrolysis. Carbonate dominates the mineralogy followed by clays and quartz. The low abundance and broad size distribution of pyrite are consistent with the presence of oxic conditions during sediment deposition. The Pearsall Formation is in the dry gas window of hydrocarbon generation (mean random vitrinite reflectance values, Ro = 1.2–2.2%) and contains moderate levels of total organic carbon (average 0.86 wt. %), which primarily resides in the inert solid bitumen. Solid bitumen is interpreted to result from in-situ thermal cracking of liquid hydrocarbon generated from original type II kerogen that was prevented from expulsion and migration by low permeability. The temperature of maximum pyrolysis output (Tmax) is a poor predictor of thermal maturity because the pyrolysis (S2) peaks from Rock-Eval analysis are ill defined. Vitrinite reflectance values are consistent with the dry gas window and are the preferred thermal maturity parameter. A Maverick Basin Pearsall shale gas assessment unit was defined using political and geologic boundaries to denote its spatial extent and was evaluated following established USGS hydrocarbon assessment methodology. The assessment estimated a mean undiscovered technically recoverable natural gas resource of 8.8 tcf of gas and 3.4 and 17.8 tcf of gas at the F95 and F5 fractile confidence levels, respectively. Significant engineering challenges will likely need to be met in determining the correct stimulation and completion combination for the successful future development of undiscovered natural gas resources in the Pearsall Formation.

Geological and geochemical characterization of the Lower Cretaceous Pearsall Formation, Maverick Basin, south Texas: A future shale gas resource?

Impact of stylolitization on diagenesis of a Lower Cretaceous carbonate reservoir from a giant oilfield, Abu Dhabi, United Arab Emirates

Movement of petroleum from the source via carrier bed to the reservoir rocks is called migration. This is divided into primary migration, defined as the movement of oil and gas through and out of the fine-grained source rocks, and secondary migration, the movement through wider pores in carrier and reservoir rocks to the trap. (Not discussed here are the aspects of petroleum entrapment, redistribution in traps and petroleum loss from traps — generally called “dismigration” — or the various types of traps.) In addition to petroleum generation, petroleum migration is the principal process in the formation of explorable petroleum accumulations (which, however, represent only a special case of petroleum migration in which an otherwise diffuse fluid flow field is highly focussed towards a reservoir structure).

Petroleum Migration: Mechanisms, Pathways, Efficiencies and Numerical Simulations

It has been long recognized that fine-grained sediments of both siliciclastic and carbonate compositions are the principal source rocks of petroleum. The common coincidence of significant, and often high, content of organic matter within such sediments, as first comprehensively documented by Trask and Wu (1930), provided the initial clue and support for this now generally accepted opinion.

Fausto Mosca

Papers

Pressure solution in carbonate source rocks and its control on petroleum generation and migration