Genetic Mechanism of Diagenetic Trap in Deeply Buried Nearshore Subaqueous Fan in Dongying Sag, East China

Dongying sag is a typical hydrocarbon-rich rift basin in East China. Large-scale Paleogene nearshore subaqueous fans adjacent to source rocks of deep lake facies developed along the basin boundary faults in the northern steep slope zone of Dongying Sag. The diagenetic trap reservoir in deeply buried nearshore subaqueous fan has been an important target for subtle trap exploration in China. However, genetic mechanism of diagenetic trap in deeply buried subaqueous fan is unclear, which inhibits the exploration and production of this type of reservoir in the study area. Research shows that different sedimentary characteristics among root fan, middle fan and fan edge of nearshore subaqueous fan in Minfeng area, Dongying Sag control the formation of deep diagenetic trap. Root fan subfacies, without the development of lacustrine mudstone, consists mainly of matrix-supported conglomerate. Compaction enhances and the primary porosity decreases sharply with depth, which leads to less development of secondary porosity. Therefore, deeply buried root fan subfacies generally with porosity less than 8% (always less than the lower limit of porosity) can be the lateral seal and sometimes top seal of reservoir. Middle Fan subfacies consists mainly of pebbled sandstones, gritstones, and deep lacustrine facies mudstones (or source rocks). With the increase of the burial depth, the compaction and cementation enhances, the primary porosity decreases gradually. Meanwhile, organic acids produced by organic matters in source rock in the process of thermal evolution can migrate into sand-conglomerate reservoir of middle fan. Organic acids dissolve feldspars, carbonate cements and other minerals to form secondary porosity. Therefore, reservoir in deep buried middle fan subfacies can still develop effective reservoir with porosity more than 10%. Fan edge subfacies, mainly consists of thin turbidite with high shale content and weak anti-compaction ability. As depth increases, compaction and cementation reduced porosity dramatically. Only in areas where overpressured fractures exist, fan edge subfacies can still develop some effective reservoirs. As a result, in nearshore subaqueous fan buried deeper than 3300m, the diagenetic traps can be developed with roof fan subfacies as lateral seal and sometimes top seal, lacustrine mudstone in fan middle subfacies as cap rock, sandy conglomerate in fan middle subfacies and limited fan edge subfacies as effective reservoir.  

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California