Paleoporosity and critical porosity in the accumulation period and their impacts on hydrocarbon accumulation—A case study of the middle Es3 member of the Paleogene formation in the Niuzhuang Sag,Dongying Depression,Southeastern Bohai Bay Basin,East Chi

Similar reservoir sandbodies and fault conduit systems in the sandstone reservoirs in the middle Es3 member of the Niuzhuang Sag have been problematic for a long time. The following problems remain unsolved： 1） The distribution of sandstone porosity is inconsistent with the hydrocarbon accumulation. The oil sandstones have low porosity instead of high porosity. 2） Sandstones, which have the same properties, have different levels of oiliness, and the sandstones with almost the same properties show different degrees of oil-bearing capacity. This study analyzes the condition of reservoirs in the research area during the accumulation period in terms of paleoporosity estimation and discusses the critical porosity of the sandstone reservoirs during the same period. The following conclusions can be drawn from the results. 1） Although reservoir properties are low at present and some reservoirs have become tight, the paleoporosity ranging from 18% to 25% is greater than the critical porosity of 13.9%. As the： loss of porosity is different in terms of burial history, the present porosity cannot reflect porosity during the accumulation period. Similar/y, high porosity during the accumulation period does not indicate that tbe present porosity is high. 2） The present reservoir location is consistent with the distribution of high paleoporosity during the accumulation period. This result indicates that high porosity belts are prone to hydrocarbon accumulation because of the dominant migration pathways generated as a result of property discrepancies under similar fault conduit conditions. Consequently, the hydrocarbon mainly accumulates in high porosity belts. Paleoporosity during the accumulation period is found to be a vital controlling factor. Therefore, high paleoporosity sandstones in the middle Es3 member of the Niuzhuang Sag have great potential for future exploration.     

乌尔逊—贝尔凹陷构造沉积充填演化对油气成藏的控制作用

乌尔逊—贝尔凹陷是海拉尔盆地中部断陷带两个主要的二级构造单元,是海拉尔盆地勘探主要领域。从构造演化各个阶段的沉积充填特征分析乌尔逊—贝尔凹陷油气生储盖的配置关系,探讨了构造沉积充填演化对不同构造带油气成藏的控制作用,分析了油气成藏模式及油气富集规律。研究表明:乌尔逊—贝尔凹陷构造演化控制着沉积充填特征,进而控制着生储盖的配置关系;乌尔逊—贝尔凹陷构造沉积充填特征决定了不同构造带具有不同的油藏类型及油气成藏模式,构建了构造沉积耦合控制油气成藏的3种模式,即复式箕状断陷凹中隆起带成藏模式、大型缓坡断阶带成藏模式和深小稳定洼槽型成藏模式。