AAPG Geoscience Technology Workshop

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Hydrocarbon-Driving Forces in Deep Tight reservoirs and Evaluation of Their Contribution to Reservoir Formation


Hydrocarbon occurrences in deep-target layers are characterized by tight reservoirs, low porosity and permeability, close association to sources rocks, widespread and continuous distribution, and absence of major buoyance force contribution. Hydrocarbon-driven forces for tight reservoir formation are generally called non-buoyance-driven with several forces being recognized relating to strata compaction, inflation of rocks and fluids, volume increases due to the transformation of Kerogen to hydrocarbons and water expelled from clays including, diffusion of hydrocarbons through water and organic network, differential capillary pressure force, and etc. Which kinds of non-buoyance-driven forces are more important and what are their contributions to hydrocarbon accumulations in deep tight reservoirs are not well understood despite of more and more tight reservoirs being discovered. Nine forces for deep hydrocarbon reservoir formation have been recognized and their contributions have been simulated based on a comprehensive dynamic model for hydrocarbon migration and accumulation. It was established by analyzing characteristics of deep reservoirs in six representative basins and conducting experiments of hydrocarbon migration and accumulation under action of major factors. It is discovered that different forces may make different contributions to the hydrocarbon migration and accumulation at different stages. Overall, (1) the most important forces are the differential capillary pressure in the tight reservoirs, which is followed sequentially by (2) volume expansion due to Kerogen conversion to hydrocarbons, (3) volume increases due to clay-expelled water, (4) diffusion of hydrocarbons through water, (5) inflation of rocks and fluids due to temperature increase. The contribution from strata compaction is quite limit as it occurs quite early during burial. Our modelling results for the corresponding contributions are ≥60%, ≈15%, ≈14%, ≈10%, and ≤1%, respectively, using geological data of deep-buried source rocks and tight sandstone reservoirs from the Tarim Basin, western China.