Characteristics And Causes Of Under-Compaction Layers Of Main Target Formations In Nanpu Sag

Nowadays,as the world's increasingly serious energy crisis,conventional hydrocarbon resources can hardly support our uses of daily life and national product.Therefore,exploring and developing unconventional hydrocarbon resources must be enforced.The key is to find relatively high porosity in the setting of low porosity and permeability.Compaction is the most important work of solidification and diagenesis of sediments and always causes porosity of sediments to reduce.Instead,under-compaction is a phenomenon in which compaction is not complete function and can preserve porosity of deep reservoirs.Thus,it can make favorable relative high-porosity reservoirs form.Meanwhile,abnormal high pressure appears along with under-compacted functions.This paper began with calculations of formation overpressure and discussed the causes of it,which can tell us distribution of under-compaction layers.And then,studying on the favorable causes of under-compaction may play a positive role of developing tight oils.In this study of mudstone under overpressure,Dongying?Ed?and Shahejie?Es?Formation of Paleogene system are widely forming overpressure and can be divided into two abnormal high pressure systems.Pressure coefficients mainly vary from 1.0 to 1.4,while the maximum pressure coefficients can reach 1.6.Meanwhile,the first overpressure system can be called low overpressure structure,which distributes from Ed to the first section of Shahejie Formation?Es₁?and another one distributes from the first section of Shahejie Formation?Es₂?to the third section of it?Es₃?with relatively high pressure coefficient.According to further research,the main cause of low overpressure system should be under-compaction.However,the causes of high overpressure system may have several different origins.For example,deep layers in the South of Gaoliu Fault may be produced by hydrocarbon generation,while overpressure of Es₃ may be the origin of under-compaction and hydrocarbon generation.Moreover,origins of under-compaction layers are effects of high sedimentation rate,fine grain size and lithologic assemblage.It also can be found that the different causes of high overpressure system in Gaoliu area may be influenced by formation erosion and depositional environment difference.