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SUMMARIZATION OF SUBNORMAL PRESSURES AND ACCUMULATION MECHANISMS OF SUBNORMALLY PRESSURED PETROLEUM RESERVOIRS

ZOU Huayao1,HAO Fang2,CAI Xunyu3(1.Faculty of Natural Resource and Information Technology, Petroleum University,Beijing 102200,China;2.Faculty of Earth Resources, China University of Geosciences,Wuhan Hubei 430074,China;3.Science and Technology Department,Southern Petroleum Explorationand Development Subcompany of SINOPEC,Kunming Yunnan 650041,China)  
The synthesizing of a great number of references on origin of subnormal pressure and underpressured pools demonstraites that two mechanisms are predominantly responsible for subsurface underpressure. The first, probably the most important one, ascribes to erosional rebound; and the second is the effect of hydrostratigraphy and topography. Other causes, such as chemical osmosis and cooling of fluid, seem to play a minor role in underpressure development. Three typical underpressured pools are classified in terms of trap geometry, accumulating mechanism associated with subnormal pressure development. The first type of conventional stratigraphic pools (except lenticular sandstone body) is sealed by low permeability rocks, with little tendency for downdip and edge water to advance, as well as with petroleum charged frequently from downdip source rocks. The second type of isolated lenticular sandstone body reservoirs,abundant in the deepcentral parts of many basins, is characterized with the full presence of oil and gas and absence of water. Several processes, including compacting, hydrofracturing and capillary phenomena prior to uplifterosion, as well as subsequent inwardshale flow driven by erosional rebound, will be able to explain petroleum charged into and water expelled out of isolated porous sandstone bodies. The third type of gas accumulations in lowpermeability reservoirs commonly is located in a basincenter and is downdip from waterbearing rocks. Underpressured gas accumulations evolve from overpressured accumulations due to regional or local structural uplift. Two casestudies, southwestern Alberta Basin and Baise Basin (southwestern China),both experiencing regional structural uplifterosion, indicate that hydrodynamicallyclosed underpressured systems facilitate petroleum accumulation and preservation.
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