Overpressure And Hydrocarbon Accumulation Of Shahejie Formation In Dongying Depression

Dongying Depression is an oil-rich overpressure depression in Jiyang Depression of Bohai Bay Basin.Overpressure has an important impact on the process of oil and gas accumulation.Overpressure mainly influence the oil accumulation in two aspects:1)Overpressured hydrocarbon bearing fluid can change the reservoir fluid environment and affects the process of reservoir diagenesis and evolution.2)Overpressure is an important driving force for oil migration and accumulation.Taking the overpressure in the Es3 and Es4 formation as the research object,the drilling data,well logging data,core sample tests and crude oil sample were used to research the generation,distribution and evolution of overpressure in Dongying Depression.And discusses the influence of overpressure on sandstone reservoir diagenesis,especially quartz overgrowth and carbonate cementation.Combined with the distribution characteristics of overpressure,the calculation method of plane residual pressure gradient is proposed,and then the law of oil and gas migration and accumulation under overpressure in Dongying sag is analyzed.The results are as following:1)The mudstone in the Es3x and Es4s member are the main source rocks in Dongying Depression.The total organic carbon（TOC）of the source rocks mainly distributed in 0.24%～5.82%.Combined with pyrolysis data,the organic matter types of the two sets of source rocks are mainly typeⅡ₁with a small part of typeⅡ₂.The analysis of organic geochemical characteristics of typical source rock samples shows that the source rocks of the Es3x have high ratio of Pr/Ph（1.04～1.43）and low gammacerane index about 0.05,while the source rocks of the Es4s have low ratio of Pr/Ph（0.33～0.52）and high gammacerane index about 0.53～0.85.The abundance of C₁₉tricyclic terpene and C₂₀tricyclic terpene in the source rocks of the Es3x are significantly higher than that of the Es4s,it shows that the organic matter of the source rocks in the Es3x are mainly freshwater lake facies deposition,and the organic matter of Es4s are salt lake facies deposition.Three types of crude oil can be distinguished in Dongying Depression according to their organic geochemical characteristics.Class I crude oil has high ratio of Pr/Ph（0.96～1.62）,low gammacerane index（0.04～0.11）and high abundance of 4-methylsterane;Class II crude oil have lower ratio of Pr/Ph（0.48～0.91）,higher gammacerane index（0.05～0.24）and higher abundance of4-methylsterane;while Class III crude oil has low ratio of Pr/Ph（0.33～0.46）,high gammacerane index（0.37～1.03）,low abundance of 4-methylsterane.Combined with4-msi,C₃₅22S/C₃₄22S,C₁₉/C₂₃TT and C₂₉ββ/（ββ+αα）and C₂₉20S/（20S+20R）biomarker parameters,it can be seen that Class I crude oil mainly are from the source rocks of Es3x,Class III crude oil mainly comes from the source rocks of Es4s,and Class II crude oil are the mixed source rocks of Es3x and Es4s.2)The numerical simulation of source rocks shows that the hydrocarbon generation and expulsion process of the source rocks of the lower and upper Es3 and Es4 in Dongying depression can be divided into two stages.And at least two stages of crude oil generation and expulsion can be identified.The early stage of oil generation is at 30～25Ma and the late stage of oil generation is at 15～0Ma.From the oil generation volume and oil generation rate,it can be seen that the oil generation volume in the late stage is significantly higher than that in the early stage.The thin section observation of sandstone inclusions in the reservoir shows that there are mainly two kinds of fluorescent oil inclusions in the sandstone reservoirs of Es3 and Es4,yellowish and blue-white.The temperature measurement results show that the homogeneous temperature distribution of yellowish fluorescent oil inclusions is between 60°C and90°C,while the homogeneous temperature distribution of bluish white fluorescent oil inclusions is between 80°C to 120°C,and the corresponding homogeneous temperature of brine inclusions in the same period is between 80°C to 140°C and 110°C～170°C,respectively.Combined with the burial history thermal maturity history map of typical wells,it can be determined that the two main periods of oil charging in Es3 and Es4 in Dongying Depression are 30～25Ma（sedimentary period of Dongying Formation）and10-0Ma（from the end of Guantao formation to the present）.3)The drilling stem test（DST）data show that the overpressure are mainly distributed in Es3x-Es4s in Dongying Depression.The maximum pressure coefficient is1.99 with an excess pressure of 39.62MPa.The acoustic travel time well log has a good response relationship with the overpressure and the acoustic travel time deviates from the normal trend with the upper member.The acoustic travel time can be used to predict the formation pore fluid pressure.Based on the logging response characteristics of overpressure and the relationship between effective stress at measured points and acoustic velocity,combined with the maturity characteristics of source rocks in Es3 and Es4 in Dongying Depression,it can be determined that the overpressure in Es3 and Es4in Dongying Depression is caused by oil generation.4)Overpressure in sandstone reservoirs in Es3 and Es4 in Dongying Depression are mainly caused by overpressure hydrocarbon bearing fluid filling.The diagenetic environment of sandstone reservoir changes due to the filling of overpressure hydrocarbon bearing fluid.（1）Two stages of quartz overgrowth can be distinguished in Es3 and Es4 sandstone reservoirs in Dongying Depression and the two stages of quartz overgrowth were formed in the two stages of oil filling.The quantitative statistical results of quartz overgrowth in thin sections show that the quartz overgrowth of sandstone in oil layers are same with its in water layers in Es3 and Es4,and primary oil inclusions are developed on the quartz s overgrowth,it indicates that the overpressure hydrocarbon bearing fluid filling in Dongying Depression does not inhibit the quartz overgrowth.Combined with thin section observation,oil test data and XRD test of clay minerals in Dongying depression,it is considered that overpressure hydrocarbon bearing fluid filling changes the p H value of sandstone reservoir,makes the reservoir acidic,reduces the solubility of silica,and causes feldspar dissolution to provide silica source for the development of quartz overgrowth.The oil saturation of sandstone reservoir in Es3 and Es4 in Dongying Depression is about 10.40%to 62.70%,that is,there is still enough non bound water in the sandstone reservoir to provide medium conditions for the formation of quartz overgrowth.（2）There are four types of carbonate cements:calcite,dolomite,ferroan calcite and ankerite in the sandstone reservoirs of Es3 and Es4.Calcite and dolomite were early diagenetic products.Ferroan calcite and ankerite were formed after the first stage of oil filling and before the second stage of quartz overgrowth.And with the increase of oil saturation,the content of carbonate cement in the sandstone reservoir decrease.Automorphic rhombic ankerite are mainly developed in oil-rich and oil immersed sandstone reservoirs.5)The excess pressure is the main driving force of oil migration in the overpressure system.The calculation results of residual pressure gradient on the top of lower Es3x and Es4s in Dongying Depression show that the plane distribution of excess pressure gradient on the top of Es3x and Es4s in Dongying Depression has the characteristics of"two low and one high".The excess pressure gradient is distributed in a ring around the sag belt,and the excess pressure gradient inside the sag belt is low,The excess pressure gradient in the fault step zone at the edge of the depression zone is high,and the outer edge is the atmospheric pressure zone,with a excess pressure gradient of 0MPa/km.Combined with the analysis of the indicated migration parameters of carbazole compounds in oil,the direction of excess pressure gradient is consistent with that of carbazole compounds compounds in oil,which can indicate the direction of oil migration in overpressure system.The area where the direction of excess pressure gradient converges is the location of preferential accumulation of oil.Combined with the distribution map of oil reservoirs in Es3 and Es4 in Dongying Depression,it can be seen that the reservoirs are mainly distributed near the fault zone in the high excess pressure gradient area at the outer edge of the overpressure center;Some reservoirs can accumulate and distribute due to the lateral migration of oil from the high excess pressure gradient area to the normal pressure area;In addition,the oil from Es4s source rocks can migrate vertically along the fault to the upper reservoir in the high excess pressure gradient area,and accumulate with the oil from Es3x source rocks in fault related traps or other types of traps to form a mixed source reservoir.