Abstract:

Petroleum exploration and research show that the small-medium lenticular sand bodies in the middle or the lower 3rd member of Shahejie Fm in the Niuzhuang subsag have different petroliferous property. Three lenticular turbidite bodies (Niu 20C3, Wang 70B and Niu 35B1)were selected as case studies,including the sand body horizon,buried depth,tectonic position,sedimentary system and facies, petrologic character, grain size, sorting and roundness, porosity and permeability, pore type,pore throat radius,diagenesis,petroliferous property, sand body characteristics,trap filling,reservoir pressure and so on. Through detailed geological analyses, it is found that the Niu 20C3 sand body has the trap filling of 74.56%,with the best conditions of hydrocarbon supply and petrophysical property;with Wang70B has better conditions of hydrocarbon supply and petrophysical property;with the medium petroliferous property (trap filling of 53.25%);Niu 35B has the lowest petroliferous property (trap filling of 29.02%),with the worst conditions of hydrocarbon supply and petrophysical property.The research results of three cases studies indicate that the main factors which control oil and gas accumulation in sand lens reservoir are sedimentary environment or genetic type of sand body, hydrocarbon supply of surrounding rock,and petroliferous property of sand body. In the turbidite sand reservoirs,the petroliferous property proportional to hydrocarbon supply condition of surrounding rock as well as porosity and permeability of sand body.The genetic type of sand body is controlled by sedimentary environment and lithofacies palaeogeograph. Sedimentary environment not only controls the reservoirs macroscopical distribution and development, but also have an important influence on lithology and microscopical feature.Turbidite sand lens reservoirs confined in source rock are favorable for petroleum accumulation, because the oil and gas sourced from mud rock can move into them from all different directions.Oil and gas can be accumulated only when the hydrocarbon-generating amount,which is transformed from organic matter contacting with sand body,execeds residual hydrocarbon amount (including the absorbed hydrocarbon,soluble hydrocarbon in porous water,soluble gas in oil and blocking hydrocarbon by capillary pressure)during the burial evolution process of source rock.Petrophysical characteristics of sand body, including porosity, permeability, grain size, sorting and heterogeneity, play especially important role in petroleum accumulation of sand lens reservoir. Experiments demonstrate that only if the grain size of lithologic reservoir exceeds some critical size, oil and gas can be accumulated in the sand body.During the geological history, the evolution of oil and gas accumulation in lithologic reservoir could be divided into three phases. The first one is in shallow depth where such a lithologic oil and gas pool does not exist because the hydrocarbon amounts,sourced from the mud rock confinding sand body,does not exceed the dispersed and residual hydrocarbon amounts.The second one is in the medium depth where the lithologic oil and gas pools could be formed because the hydrocarbon-generating amount is enough to supply the oil and gas accumulation in lithologic pool and organic acid formed by organic matter transformation might improve the reservoir petrophysical property.The last one is in a great depth where oil and gas can not be accumulated owing to the poor petrophysical property of reservoir.Therefore,as the burial depth is increased,the lithologic pool formed by stages,petroleum is only accumulated in the second phase with medium depth.

Key words: Bohai Bay basin, sand lens reservoir, main controlling factors, petroleum accumulation mechanism and model