The effects of petroleum retention within and expulsion from six intervals within the Triassic Chang 7 Member in the Ordos Basin in China have been demonstrated by deciphering a large suite of petrologic, organic petrographic, and organic geochemical analyses on 106 shale samples.

Organic properties and lithological heterogeneities control the composition and the amount of retained petroleum. Enrichment of aliphatic hydrocarbons in the silty, organic-lean intervals versus enrichment of aromatic fluids in the clayey, organic-rich units are probably caused by compositional fractionation occurred during intrasource (primary) and/or short-distance secondary migration. This speculation was further supported by the negative expulsion efficiencies calculated using mass-balance models in the organic-lean intervals, indicating excess petroleum have migrated and accumulated therein.

The most important implication of this study is that the hydrocarbons in the 1st and 4th intervals constitute potential petroleum exploitation targets, given (i) improved oil quality (more aliphatic oil), (ii) lower sorption affinities, (iii) relatively high degree of brittleness in these two intervals. Despite their high contents of retained hydrocarbons, organic-rich 5th and 6th intervals are not best targets, mainly due to the high sorption affinities of organic matter structure thereby causing low oil mobility and producibility.


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