Origin And Evolution Of Lowly Permeable Sandstone Reservoir Bed And Its Influence On Oilfield Distribution

Lowly permeability sandstone reservoirs play a significant part on Chinese hydrocarbon exploration and development activity.Owing to quickly change of the sedimentary and diagenetic facies,strong heterogeneity, the lowly permeability reservoirs characteristiced by lowly porosity, lowly permeability, low oil saturation, small reservoir thickness, bad separation of oil and water. Thus, it has important theoretical and more practical value in analyzing and evaluating the origin and evolution of low permeability sandstone reservoir and its influence on oilfield distribution. Lowly permeability sandstone reservoirs where developed in Chang 2 division of Zhang-han oilfield is the ideal study area for research on origin of lowly permeability sandstone reservoir and its influence on oil and water distribution. The region was taken as the key object to analyse in this paper, the origin and evolution of low permeability reservoir and oilfield distribution were researched from the macroscopic and microcosmic perspective based on refined description of its macroeconomic distribution and microscopic composition. All research based on the core observation, and the influence of lowly permeability reservoir macro geometric shape (plane and vertical) on the oilfield distribution was researched by the comprehensive utilization of logging, well testing, producing test and the reserves calculation. The affect of physical property and its evolution to oilfield distribution was researched by micro-pore structure analysis, cast thin section observation, scanning electron microscopy and other test method and material. The key factors that control the distribution of oilfield were emphatically analyzed combing the research of oil-water propertie variation in low permeability reservoirs, finally, the law of oilfield distribution that influenced by the origin and evolution of lowly permeability sandstone reservoir bed was summarized on the basis of the study.The result of research show that:(1) Lowly permeability reservoir characteristic with special geological reservoir features, as almost no oil layer developed in poor physical properties and lowly permeability sandstone reservoir, and the thickness of oil-water layer mainly distribute from 2 to 5 m,the types of reservoir be controlled by synthesis influence of sedimentary facies and diagenesis, It's easier to occur the oil/water inversion within the reservoir; (2) Lowly permeability sandstone reservoirs is connected and outspread sandstone bed that be composed of multilateral and vertical superposition of a ribbon and elliptic sand body unit at different phases. And its plane and interlayer heterogeneity are stronger owing to the division of isolated layer; (3) The lowly permeability sandstone of Zhang-han oilfield characteristic with finer grain size, lower mineral and compositional maturity compare to convention sandstone, is the material base of the compacted lowly permeability sandstone, and go against the formation and preservation of the pore through the initial sedimentary environment to later evolution; (4) The secondary dissolved pores is the main pore spaces in lowly permeability reservoir, and combined with residual intergranular pores and micro-porosity formed two kinds of pore compile including dissolved-residual pore type and dissolved-micropore type. Influenced by microscopic heterogeneity, the porosity is in normal distribution, and the permeability is in non-normal and logarithm distribution. And the lower permeability value is, the correlation between porosity and permeability is poorer; (5) The diagenesis degree is strong in lowly permeability sandstone of Zhang-han oilfield, the types of diagenesis is various, compaction is the main factor that reduced the reservoir quality of the sandstones.cementation is less important factor, dissolution contributes more to physical property,In four diagenetic facies formed in sandstone pass through different diagenesis, predominant sandstones with these diagenetic facies are belong to level of ultra-lowly permeability; (6) Micro structure and strong inner heterogeneity are main macroscopical factors that contribute to the formation of lowly permeability reservoirs under the background of low permeability; Higher bound water saturation (20% to 40%), larger thickness of oil-water transitional zone, directional permeability are microscopic factors that perplex the distribution of oil and water within Low-permeability reservoir.