Beyene Girma Haile

Diagenesis and porosity-permeability evolution of low permeability reservoirs: A case study of Jurassic Sangonghe Formation in Block 1, central Junggar Basin, NW China

Abstract Based on core observation, thin section examination, cathode luminescence analysis, scanning electron microscopy, fluid inclusions, carbon and oxygen isotope, mercury penetration, porosity-permeability test and other analytical methods, combined with the histories of burial evolution, organic matter thermal evolution and hydrocarbon charge, the diagenesis and porosity-permeability evolution are studied of low-permeability reservoirs of Jurassic Sangonghe Formation in Block 1 of central Junggar Basin. The matching relation between reservoir porosity-permeability evolution and hydrocarbon accumulation history is analyzed. The diagenetic environment evolution of the reservoir in the study area is early alkaline, interim acid and late alkaline, forming the diagenetic sequence of chlorite membrane precipitation, early calcite cementation, feldspar dissolution accompanied by quartz overgrowth and authigenic kaolinite precipitation, anhydrite cementation, late period ferrocalcite and ankerite cementation, a small amount of pyrite cementation. Generally, compaction occurs throughout the whole burial process. According to the matching relation between reservoir porosity-permeability evolution and hydrocarbon accumulation history, the Jurassic Sangonghe Formation has three genetic types of low permeability reservoirs: densification after hydrocarbon accumulation, with the best exploration potential; densification during the hydrocarbon accumulation, with medium exploration potential; densification before the hydrocarbon accumulation, with the poorest exploration potential.

Fracture Patterns in Organic Rich Mudstones - Implications for Primary Petroleum Migration

Scanning Electron Microscopy (SEM) of thin sections taken from organic rich Jurassic shale core samples from the Norwegian Continental Shelf reveal the existence of what appear to be fine scale fracture patterns filled with migrated petroleum oriented normal to depositional bedding. The suggested fracture patterns forms in shales rich in fine grained clays (smectite, kaolinite) at temperatures around 90 sC (2500m burial depth). Coarser grained shales appear not to be fractured on a thin section scale. Fewer and less extensive micro fractures are found in samples intermediate between the fine and coarse grained samples investigated. The mechanism responsible for the fractures is suggested to be pressure build up during initial maturation around isolated patches of organic material before a functioning migration network has been formed. This indicate that a certain amount of petroleum must be produced before an effective long range organic network functioning as a migration pathway is established in source rocks. The absence of fracture networks in coarser grained source rocks indicate that functioning migration networks are establish in such rocks before local fracture pressure is reached. This is probably due to higher permeability resulting in a more effective displacement of the continuous porewater phase.