Characteristics and Formation of Intrusive Diabase-Exomorphic Pelite Reservoir from Northern Slope of Gaoyou Sag, Subei Basin of Eastern China

Abstract Organic matter-rich mudstone or shale from Funing formation of Gaoyou sag is altered by concordantly emplacement of diabase: the impermeable mudstone or shale closely contacting to diabasic intrusion is transformed... [ view full abstract ]

Abstract
Organic matter-rich mudstone or shale from Funing formation of Gaoyou sag is altered by concordantly emplacement of diabase: the impermeable mudstone or shale closely contacting to diabasic intrusion is transformed into hydrocarbon reservoir rock due to metamorphism. Meanwhile, organic matter maturity of country hydrocarbon source rock is significantly elevated and hydrocarbon generation is accelerated under the effect of massive heat brought by diabasic intrusion. Intrusive diabase and its related exomorphic pelite form a complex reservoir system. Reservoir spaces of diabase are formed due to cooling and secondary alterations; mudstone or shale contacting to intrusion is transformed into slates or hornfels in responding to different distances and various reservoir spaces are produced under the influence of diabase. Both diabase and exomorphic pelite are characterized by rather poor petrophysical property (low permeability and porosity) and strong heterogeneity. The fracture system formed during late-stage metamorphism in intrusive diabase-exomorphic pelite reservoir is the crucial factor for enhancing reservoir quality: intermediate metamorphism zone (hornfels) shows more favorable reservoir quality than low degree zone (slates) because hornfels are closely contacting to diabase and more intensely affected by heat, thus (micro-)fractures are more abundant in hornfels; reservoir quality of diabase edge is more favorable than diabase center because the edge cools down more quickly and crystallizes disorderly, thus developing more cooling fractures and primary vesicles. The non-correlation between permeability and porosity in diabase and exomorphic pelite indirectly testifies the existence of fracture system, which plays an important role as reservoir spaces and seeping channels as well as connection of other reservoir pores. The development of exomorphic pelite reservoir is mainly controlled by diabasic intrusion but not influenced by compaction; the diabase controlling mechanism includes thermal baking, hydrothermal fluid expelling, dissolution and mineral contraction. The reservoir quality of exomorphic pelite is also influenced by clay content and distance from diabasic intrusion.
Diabasic intrusion has accelerated organic matter maturation and its transformation into hydrocarbon, which is proved by the characteristic geochemical index and uneven distribution of hydrocarbon prospects. Meanwhile, petrologically unaltered mudstone shale and hydrocarbon generation induced overpressure act as reservoir cap. Thus an intrusive diabase-exomorphic pelite reservoir system is possibly formed since source rock, reservoir rock, cap rock and migrating pathways are fulfilled.
Keywords: Northern slope of Gaoyou sag; Funing formation; Diabase intrusion; Metamorphic reservoir; Reservoir formating mechanism