Overpressure Distribution And Evolution,Dynamical Mechanism Of Oil And Gas,Dongying Depression

Overpressure has an important impact to many aspects of the hydrocarbon accumulation process, such as hydrocarbon generation, migration and accumulation. Overpressure distribution characteristics and evolution is an important aspect of petroliferous basin geological research, the study also has a great significance for the drilling engineering and oilreservoir engineering. The source rocks of Es3and Es4in Dongying Depression, development of large-scale overpressured systems that as a research object, the use of drilling, well logging, seismic, and test analysis data, the basis of the results of our predecessors have been studied, Dongying Depression today overpressure distribution characteristics, origin and evolution, and discusses the differences of the different pressures environmental conditions, oil and gas drive mechanism and its oil and gas migration and accumulation. The results have obtained as follows:(1) Dongying Depression is a typical overpressured depression with abundant petroleum resources, a large-scale overpressured system occurs in the third and fourth members of the Shahejie Formation in the depression which evidenced by drill stem test (DST) data, and the source rocks are also existed within the overpressured system. Based on the2400DST data from1485wells, the burial depths of overpressured sandstone reservoirs range from2200to4400m with excess pressures of about3.4to39.6MPa and pressure coefficients of1.2to1.99. According to the integrated interpretation of drilling, well logging and seismic velocity data, the large-scale overpressured system is associated with significantly increase in drilling mud density, as well as abnormal high sonic transit times with deviation from the normal trend and low seismic interval velocity responding. By the integrated explanation, the burial depths of the top overpressured surface range from2200to2900m, corresponding formation temperatures ranging from90to120℃, the depths of top overpressure surface increase as the burial depths of source rocks deepen, the characteristics and magnitudes of overpressured system are coincident with the cumulative thickness and burial depth of mature source rocks, and thermal degradation oil-generation of source rocks. The increase of high sonic transit times and low seimic interval velocity responding can be used to predict the distribution of large-scale ovpressured system. This study indicates that the large-scale overpressured system is well developed where the cumulative thickness of mature source rocks is thick and source rocks remain in oil-generating stage of thermal evolution with vitrinite reflectance values of overpressured source rocks various from0.5to1.3%, and the fault system and sandstone transporting layers have an important influence on the distribution and construction change of the large-scale overpressured system in the third and fourth members of the Shahejie Formation of Dongying Depression.(2) The fluorescence color of oil inclusions are mainly yellow-white (medium maturity) and blue-white (high maturity), fluorescence spectrum peak wavelength characteristics can be divided into two types, yellow-white fluorescent and blue-white fluorescent oil inclusions homogenization temperatures range respectively from70℃to100℃and from100℃to130℃, and corresponding to the same period in aqueous inclusions of the homogenization temperature range from100℃to140℃and from140℃to180℃. The timing of oil charge by combining the homogenization temperatures of the aqueous fluid inclusions coeval with the oil inclusions with the inferred burial and geothermal histories of the host rocks, On the basis of the integration of the geohistory modeling results and the fluid inclusion homogenization temperature data, two episodes of oil charge are identified in the Es3and Es4reservoirs of the Dongying Depression at around the end of Ed depositional period (30～25Ma) and the Nm and Ng depositional period (8～0Ma), respectively. Different fluorescence colors of oil inclusions may be cause for the same period of the oil and gas injection, composition difference is mainly caused by the oil and gas from different sources. According to the source rock maturation history and hydrocarbon generation and expulsion history of the simulation results, in the second oil charge period, kerogen conversion rate and amount of hydrocarbon generation and expulsion is a substantial increase in the Es3x and Es4s formations, so it is the most significant oil charge period in Dongying Depression. In the first and second oil generation and charge period, the overpressure development was detected by thermodynamic modeling of fluid inclusions. In accordance with the conclusions of the test oil in the wells that reservoir and oil-water bed development overpressure, while the development of normal overpressure of the water layer, so the origin of overpressure in the sandstone is transfer of the overpressure from source rocks.(3) According to the different phenomenon of effective character which is controlled by origin overpressure mechanism of compaction disequilibrium and oil generation, the effective stress is calculated from the DST data and used to analyze the origin of overpressure. According to the results of the measured formation pressures from Drilling Stem Test (DST), sonic logging and density logging data, correlated the graphs of measured pressures and effective stress verses the depth, the origins of overpressures in the depression are both compaction disequilibrium and oil generation, this studies indicate that:①there are apparently separation differences of measured pressures and effective stress between undercompaction and oil generation. In the process of compaction disequilibrium, as a vertical sediment loading increases during burial, the pore fluid expulsion in shales is blocked and porosity reducing is not consistent with subsidence and rapid deposition, leading to the overpressures to occur due to the pore fluid undertaking a part of overburden stress. It seems that the effective stress increase is restricted, the overpressure trend is coincidence with the increase of overburden stress, and the values of effective stress verses the depth is about a constant in this condition. However, when the overpressures are formed due to oil generation within the normally compacted shales, the degree of overpressures is not influence by lithostatic pressure on the overlying, and the trend of excess pressure increase with depth is not consistent with the overburden stress increase, the effective stress relating to the overpressures with the regime of oil generation is different from the overpressures caused by compaction disequilibrium;②using the relationship of effective stress and sonic velocity, the loading curve and the unloading curve can be described to explain the regimes of the overpressures caused by undercompaction and oil generation. Based on the calculated effective stresses using the measured data, the origins of overpressures in Dongying Depression are considered to be compaction disequilibrium and oil generation, but the overpressures caused by undercompaction are small with the relative shallow depths, and mainly distributed in the Lijin sag North with Niuzhuang sag eastern region. Oil generation is the important regime for the formation of deep basin-scale overpressures.(4) The results of numerical simulation method combined thermodynamic modeling of fluid inclusions indicate that the origin of overpressure in the first phase is mainly compaction disequilibrium.①Quantitative estimation of overpessure cause by oil generation shows that the the overpressure value cause by oil generation is difficult to reach the rupture stress levels in the mudstone, and this value less than the thermal dynamics simulation of the fluid inclusions. Therefore, the pure oil-generating role as the overpressure causes is difficult impact on the discharge of crude oil from source rocks;②According to the deposition rate calculation, the Es3deposition period experienced rapid deposition process in the first phase of generating overpressure, the deposition rate can reach800-1000m/Ma, and the thick layers of mudstone occur compaction disequilibrium in Es3x and Es4s is provide favorable conditions for generating abnormal high pressure. Therore compaction disequilibrium began to form overpressure in a period of Es3formation during the deposition and earlier than the first phase of the Petroleum generation and charge, the depth range from1000m to2000m.(5) Source rock maturation history, and expulsion of the hydrocarbon generation history modeling and fluid potential calculation results show that:①The maturity (Ro%) values of main source rocks in Es3z and Es3x layers range from0.3%to1.0%, while those of Es4s layer range from0.5%to1.1%, and it is lower in the shallow edge area than in the deep depression, where the source rocks are supposed to generate mass oil at present;②The generation and expulsion processes can be divided into three stages, and there are at least two periods of oil generation and expulsion, the first one occurring from30Ma to25Ma, the second one occurring from16Ma to OMa, which is more important in the mean of oil generation and expulsion, particularly from5Ma to the present (the end of Guantao Formation in Neogene);③Simulation results show that around the edge of the sag areas and the central fault zone is favorable for hydrocarbon accumulation area, oil and gas gathering in the central fault zone in Es4s through the fracture and transporting sand for the vertical and lateral migration. On the whole, the oil from the high oil potential in the three-dimensional space to the low oil potential migration and aggregation.(6) According to the study area more than2,000wells in the measured pressure and the oil field water chemistry data, combined with the reservoir overpressure evolution, the use of the genesis of the water chemistry to study the impact of the overpressure system evolution process of formation fluid migration and accumulation, that is fluid migration features under the influence of the overpressure. The result are showed as follows:①In the normally pressure system (from Es2to Qp), which the depth above2200m, the total dissolved solid lower than100g/l, it belongs to the salt water areas; In the overpressure system (Es3and Es4), which the depth under2200m, the total dissolved solid higher than100g/l, up to336g/l, it belongs to saline areas;②In the east of the Dongying Depression, the total dissolved solid is higher than west, and higher in the north than south, in Es2and Es3, the distribution of high total dissolved formation water along the fault zone, and in Es4, the distribution of high total dissolved formation water as the center with north of Dongying Depression, was reduced to outside as zonal;③The relationship between plasma and assemblages shows:in the condition of the matched relationship between the formation water and overpressure system, the high total dissolved solid formation water maybe from the salts dissolved of evaporate formation in Es4, in the evolution process of the water chemical field, the CaCl2formation water have high total dissolved solid is formed by the water-rock interaction;④In Es2and Es3, high salinity formation water is mainly the result of mixing of high salinity Es4formation water vertical migration along fault systems.(7) The end of Ed deposition from Es4s source rocks of oil and gas migration and accumulation process. The most important oil and gas charge reservoir of hydrocarbon migration and accumulation since the late Guantao group (5Ma). Analysis of the energy conditions (fluid potential of hydrocarbon migration simulation), power and resistance as well as the drive mechanism, although Dongying Depression is the oil and gas-driven system with overpressure system, in the overpressure system, hydrocarbon migration and accumulation characteristics of the transition pressure and normal pressure systems existence of significant differences. Abnormal high pressure in the overpressure environment influence and control the direction of hydrocarbon migration, migration distance and the degree of hydrocarbon accumulation to a large extent. The reservoir in the boundary and top of overporessure, weak overpressure or pressure transition zone (pressure coefficient<1.4) and pressure relief area or the normal pressure system which have the realation with the overpressure body system is favorablethe location are hydrocarbon migration and accumulation, high overpressure system (reservoir pressure coefficient>1.4) are generally not conducive to oil and gas charge and enrichment.