IMPACT OF DIAGENESIS ON RESERVOIR‐QUALITY EVOLUTION IN FLUVIAL AND LACUSTRINE‐DELTAIC SANDSTONES: EVIDENCE FROM JURASSIC AND TRIASSIC SANDSTONES FROM THE ORDOS BASIN, CHINA

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Abstract
The reservoir quality of Jurassic and Triassic fluvial and lacustrine‐deltaic sandstones in the intracratonic Ordos Basin is strongly influenced by depositional facies and various types of diagenetic modifications. The fluvial sandstones have higher average He‐porosity and permeability (14.8% and 12.7 mD, respectively) than the deltaic sandstones (9.8% and 5.8 mD, respectively). In addition to extensive mechanical compaction, eodiagenesis (220–97 Ma; depth < 2000 m; T < 70°C) has resulted in dissolution and kaolinitization of detrital silicates in the Jurassic fluvial sandstones, and in smectite infiltration and minor cementation by calcite and siderite in the Triassic fluvial and deltaic sandstones. Pervasive eogenetic carbonate cementation (> 20 vol.%) occurred in Triassic deltaic siltstones and very fine‐grained sandstones which are closely associated with organic‐rich mudstones. Mesodiagenesis (97–65 Ma), which occurred during rapid subsidence to depths of 3700–4400 m, resulted in the albitization of plagioclase, checmical compaction, the conversion of kaolinite into dickite, and cementation by quartz overgrowths, chlorite, illite, ankerite (δ13CVPDB=−2.4‰ to +2.6‰; δ18OVPDB=−21.5‰ to −10‰) and calcite (δ13CVPDB=−4.7‰ to +3.7‰; δ18OVPDB=−21.8‰ to −13.4‰). Oil emplacement (95 Ma) retarded cementation by mesogenetic quartz and carbonate but had little influence on dickite, illite and chlorite formation. Retardation of quartz cementation was also due to the presence of chlorite fringes around detrital quartz grains. Dickitization of eogenetic kaolinite together with the short residence time at maximum burial temperatures (105–124°C) has retarded the albitization of K‐feldspars and illite formation and hence prevented severe permeability destruction. Telodiagenesis, which occurred after uplift (Eocene to end‐Neogene), caused slight dissolution and kaolinitization of feldspars.This study demonstrates that despite complex patterns of diagenetic modifications in the Triassic and Jurassic successions, depositional porosity and permeability are better preserved in fluvial meandering channel sandstones than in deltaic sandstones. These results should be important for modelling of reservoir‐quality distribution and exploration risk evaluation in the basin.

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