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Geochemical Factors Controlling the Phase Behaviour of In-Situ Petroleum Fluids in the Eagle Ford Shale

Abstract

In shale plays the in-place GOR does not necessarily correspond to produced GOR because of fractionation during production. The first step in geochemically unravelling the fractionation phenomena occurring in both play types is to determine the bulk composition of the petroleum that is generated in the shale as a function of organic facies and maturity. This is because all subsequent processes, both natural and induced during production, simply act upon and modify the original composition. The Eagle Ford Shale, organic-rich and containing marine Type II kerogen, is the largest unconventional oil producer of the USA. Here we assess the in-situ bulk composition of petroleum in that shale with due consideration of organic richness, organic matter type, and structural response of the organic matter to thermal stress by evaluating the geochemical characteristics of forty seven samples that comprise a maturity series, and then performing artificial maturation (Phase SnapShot) experiments. A regional PVT database (117 reports) was used for calibration and cross-checking of predictions from the experiments. With increasing maturity the organic matter in the Eagle Ford maturity series retains more and more oil as evidenced by an increasing OSI up to a Tmax of 455°C. High retention values can be seen over a broad range of maturity levels (440-475°C). Highest maturity samples show a decreased retention capacity. Oil quality (S1/(S1+S2ex) increases concomitantly with progressive maturation. By analysis of the “instantaneous” fluids generated over narrow maturity ranges, we were able to reproduce the phase envelopes of unconventional petroleums produced from the Eagle Ford. Our conceptual model comprised two reactive components, (A) an intimate mixture of kerogen plus bitumen which generated petroleum within the low permability shale matrix, and (B) bitumen alone which was the precursor of gas in zones of enhanced porosity within the shale. The enhanced generation of gas by the reactions between kerogen and bitumen, the addition of gas from bitumen alone, and the significant retention of C7+ fluids were all crucial to matching the compositions and phase behaviour reported in the field. The model predicted near identical compositions to the target well and its physical properties (Psat, cricondenbar, cricondenthem, Bg) which also fitted very well into the general area around the chosen target well.