Study On The Overpressure Characteristic And Hydrocarbon Accumulation In Bonan Depression,Zhanhua Subbasin

Bonan sag is a prolific hydrocarbon sag in zhanhua depression. Overpressured source rocks are still capable of oil generation. Hydrocarbon accumulate in structural traps in the overlying normal pressure sediments, and oil preservation in stratigraphic traps are mainly in the overpressured system. The migration from the compartments in an oil and gas phase is a pressure-driven process in which the flow direction is controlled by the configuration and internal pressures of the fluid compartments in the eocene third and fourth members of shahejie formation (es3and es4). Greater awareness on the subjects and better understanding of the interaction between geological processes and overpressure has helped in better forecast stratigraphic traps and petroleum exploration. Based on their overview paper of porepressure characteristic, by assessing present-day pressure distribution and response based on direct pore pressure measurements, well and seismic date, as well as result from laboratory test. This paper discuss the characteristic and distribution of overpressure, and the relative contribution ratio of different porepressure generating mechanisms. The original of overpressures and development of dynamic barriers within the zone are all diagenetically driven in formation. Pressure preservation prevented overpressured migration is the result of a series of diagenetic events that determining mechanical and chemical compaction overlapped in time. The driving force in different pressure environment and geological factors affected petroleum accumulation are discussed throught the dynamic theory. The following conclusions can be drawn.1. porepressure characteristic, prediction and mechanism(1) the present-day overpressures in bonan sag commonly occurs at depths between2300m to4200m based on2981dst date from456wells. At shallow to moderate depths of burial within normal fluid pressures. Formation test indicates the maximum pressure coefficient is1.85, and intensity pressure which pressure coefficient exceed1.5below a depth of3000m. By assessing the response of present-day pressure from wells using drilling stem test (dst), wire logging and geological date, characteristics and geological affection factors are discussed in detail.①sonic transit times of mudstones deviate from the normal compaction trend line to the overpressured area revealed by drilling, but density has no obvious response.②beside the low amplitude overpressure system distributed in esl which caused by disequilibrium compaction observed. Two overpressure systems divided by salt layers of the upper part of es4have been identified, including an upper reservoir overpressure with main part in lower es3, a lower reservoir overpressure in the upper part of es4.③in bonan sag, the onset of overpressure range from2950to3150m, and the depths increase from west to east, which mainly due to the burial depth of source rock and sand content.(2) according to the response characteristics of abnormal high sonic transit times with deviation from the normal trend and low seismic interval velocity,3439pre-stack time migrations (pstms) were used and converted into interval velocity. The velocity of vertical seismic profile (vsp) from5wells, which shows more resolution than the conventional velocity data from seismic velocity spectrum, can be used to calibrate a velocity vs. Depth profile. The optimized and calibrated velocity data, in conjunction with the fillippone formula pressure prediction model, have been used with considerable success in bonan sag. The3d pore pressure modeling offers a comprehensive picture by showing the overall variation in3d view and reveals a comprehensive understanding of the pressure distribution at any direction slice. After constrained by vsp calculated velocity, the calculated interval velocities can be validated, thus lower the uncertainty from source data. According the model of bonan area, we obtain each maximum and minimum velocity from the trend line of seismic velocity spectrum, for predicting pore pressure on the basis of a deviation from a normal compaction trend in shallow area. This has been found to yield pressure vs. Depth profiles, at a bit level, with more accurate speed approach for prediction. After application to pore-pressure corrected by tying speed correction coefficient and existing well control date, nearly82percent of the statistical data has a positive or negative error in10percent of an allowed interval.(3) by assessing the presure characteristics of neocene, wire logging and pressure testing as well as geological date, several methods have been used to enable determination on overpressure origin in the sag. We concluded that overpressure are primarily generated by any conventional fluid expansion mechanism, and the role of disequilibrium compaction is unimportant, main evidence more than that including:(1) the fluid pressure of different types vs. Depth plots reveal that the overpressured reservoirs are predominantly oil saturated or oil-bearing which resulted in hydrocarbon fluid recharge with overpressure, and overpressured water reservoirs are rare. Overpressure in the sandstones is generated by pressure transmission resulting from the overpressured fluids which expel from the source rocks of es3and es4formations in bonan sag charging into the reservoir rocks;(2) calcareous mudstones in the es3formation can reduce the seal rock porosity and permeability to form a mudstone pressure seal. Overpressured source rocks are still capable of oil generation. Gas-to-oil cracking in shales in deep zone of es4formation obviously reveal a contribution of natural gas on overpressure generation;(3) natural microfractures are widespread in the low-permeability source rocks of es3and es4formations in bonan sag. Indicates oil expel from source rocks by means of active faulting and fracturing;(4) qualitative analysis of paleopressure evolution thought basin modeling technology show that the early phenomenon of overpressure was caused by disequilibrium compaction. Stress changes rapidly dissipate large amounts of overpressure in tectonically active period. The later period of overpressure is caused by fluid expansion;(5) relationships between effective vertical stress and sonic velocity of shale, as well as density and neutron porosity of shale reveal that extra high (pressure coefficient typically higher than1.5) and most moderate overpressures (pressure coefficient in the range1.3to1.5) follow an unloading curve contributed by oil generation, and part of moderate to low intensity overpressures (pressure coefficient in the rang1.1～1.3) follow an loading undercompaction curve range. After comprehensive study, we quantify a more sophisticated model based on the relationships between effective vertical stress and velocity of shale to differentiate between overpressure that arises from disequilibrium compaction and fluid expansion mechanisms. The contribution ratio increases along with porepressure coefficient. Nearly60percent of the pore-pressure in an extra high pressure coefficient (typically higher than1.5) is contributed by hydrocarbon generation with an accurate increasing of nearly15-25mpa.(4) three superimposed overpressure systems have been identified in bonan sag. Including a low amplitude overpressure system distributed in esl which caused by disequilibrium compaction; a upper reservoir overpressure with main part in lower es3; a lower reservoir overpressure in the upper part of es4. multiphase fluid stem from mud-rich sources rock compose material basis of an overpressured system. Normally compacted shale within esl, and cemented sandstone interbeded with mudstone in upper es3z/x, as well as mudstone and gypsum beds distribution in the upper part of es4s, formed respectively barrier of each system. The continuous distribution and sealing ability of of caprorks, as well as fault structure, which shows a complex and duality fault-fluid flow behavior on formation pressure and distribution of oil, control the characteristic and construction change of overpressures system.2. the diagenesis and fluid characteristics in the sandstones of overpressured strata(1)753thin sections were examined using optical microscopes for petrographical and microstructural study. The lithology of the overpressured es3, es4of bonan sag is mainly classified as fine sandstones, generally consists of lithic arkoses and litharenite. Semi-quantitative estimations of the mineralogical contents show that quartz take up20～62%of the rock volume; feldspar amounts9～40%, in which the potassium feldspar and anorthose have equal shares; lithic fragment content is8～63%, including volcanic detritus and argillaceous detritus; the matrix including argillaceous and microcrystalline silicon, ranges2～10%. The sandstones experienced severe mechanical compaction:the quartz and feldspar particles are linear contacted, the plastic mineral grains are compressed and deformed, and there is pressure solution on the quartz grains. The cements of the sandstones take an average proportion of17.5%, including carbonates, clays, authigenic quartz and feldspar, etc. The metasomatism of carbonates to feldspar is common. There are two episodes of pressure solution in the overpressured strata, represented by the feldspar pressure solution and carbonate pressure solution. The diagenetic sequence and the stages of diagenetic evolution are analyzed through the studies of the rock textures, the authigenic minerals, clay minerals, and organic matters’maturity.(2)859coulple formation water testing data suggest that the formation water of research area is mostly heavy sodium carbonate type (nahco3), and the chloride-calcium type (cacl2) takes the second place. The formation waters with mineralization degree greater than20g/l were found in the es3and es4with the depth under3200m. The ion contents display a step-increase along with the increase of depth. Three hydrogeological environments could be discriminated by the formation water ion content and their combination coefficient. The first environment is intense alternating area of oilfield water, it ranges0-1300m underground (the bottom of nm); the second is weak alternating area of oilfield water, ranges1300-2840m underground (the top of the overpressured system of es3). The mineralization, the concentration of na++k+, cl-, and the coefficient of salinization generally remain unchanged while the depth is increasing, the concentration of mg2+, so42-increase slightly; the last environment is alternating blockage of oilfield water, deeper than2840m. In this kind of environment, the mineralization, the concentration of na+k+, cl-, the coefficient of salinization, and cation exchange adsorption coefficient increase with the depth, and the values pick at3673m, where the strong overpressure exists. The interface of the change of hco3-, cl_/na coefficient is determined2000m (the regional unconformity surface of ed). The interaction of water field and the overpressure system, together with the hydrocarbon generation, determined the water-rock interaction, the inorganic-organic interaction, the fluid alternating, and the migration and reservoiring of oil and gas.(3) based on the observation of80oil inclusions and156brine inclusions from24sandstone. The homogenization temperatures of the fluid inclusions in secondary minerals can be grouped in three ranges:65～75℃,75～100℃, and100～120℃; the homogenization temperatures the co-exist saline inclusions range in80～90℃,105～115℃, and125～150℃, respectively. The fluid inclusion homogenization temperatures, combined with the strata burial history and thermal history, indicate that there are three charges in the sandstone reservoirs of es3and es4in bonan sag:the first charge took place34-24ma ago, with a small charge volume; the second charge was16-12ma ago, characterized by the low salinity of charged fluid (0～4wt%nacl); the last charge till was5-2ma, with high salinity up to2～20wt%nacl. According to the quantitative x-ray diffraction analysis of29sandstones and c, o isotope temperature conversion of authigenic carbonates in74detrital rock, the diagenetic temperature of carbonates in calcareous sandstones ranges27.5～32℃, that of the carbonates in mudstones ranges39～69℃; the diagenetic temperature of early phase calcite cements in sandstones ranges73.5-91.3℃, while the diagenetic temperature of late phase calcite cements ranges100.9～136.7℃, averaged117℃.(4) the authigenic carbonate minerals in sandstones can be divided in two types:type Ⅰ are the early phase carbonate cements, with δ13cpdb in-2～+4‰,δ18opdb in-15.02～-10.3‰; type Ⅱ are the late phase carbonate cements with δ13Cpdb<-2‰,δ18Opdb in-11.13～-17.83‰. The value of c isotope and o isotope has a positive correlation in type Ⅱ carbonate cements, demonstrating that the source of carbon has close relationships with diagenetic temperature, i.e. The generation and expulsion activities motivated by the increasing temperature of source rocks. The ratio of calcite and dolomite is different in the two types of carbonates, and the features under microscope differ as well. The type Ⅰ displays orange red under cathode luminescence, while type Ⅱ displays dark red. Carbonate cement cathode luminescence intensity indicates the changing of diagenetic fluid. The girdle structure under cathode luminescence suggests the carbonate cements were affected by the multiphase mn, fe ions and fluids of different salinity.3. petroleum accumulation in different pressure environment(1) maturity and hydrocabon generation, explusion histories modeling of source rock in bonan sag indicate:①the maturity (ro%) values of main source rocks in es3x and es4s layers range from0.5%to1.3%, where source rocks are still capable of oil generation presently;②petroleum generation and expulsion processes can be divided into two stages, the first stage occurring from32ma to25ma, the second stage ranging from16ma to Oma is more important for mass of oil generation and expulsion, especially from5ma to the present (the end of guantao formation in neogene);③simulation results show that the north edge of the sag areas is favorable for hydrocarbon accumulation. Petroleum originate in es4s source rock migration and accumulation process happened in the end period of ed deposition. The most important epoch of petroleum migration and accumulation is dominated by the es3x source rock at the late guantao group (5ma). On the whole, the oil migration from the high potential area in the three-dimensional space to the low potential area and accumulation within5ma.(2) according to energy condition of petroleum migration, etc. Fluid potential of hydrocarbon migration simulation, driving force and drag force as well as driving mechanism are discussed in detail. The migration from the compartments in an oil and gas phase is a pressure-driven process in which the flow direction is controlled by the configuration and internal pressures of the fluid compartments. Many sedimentary basins contain layers of two or more superimposed hydrogeological systems. The shallow systems are usually basin wide in extent and exhibit normal hydrostatic pressures. The deeper systems, where the oil is generated, are not basin wide and are abnormally over pressured. They usully consist of a series of individual fluid compartments that are not in hydraulic pressure communication with each other nor with the overlying hydrodynamic regime. The driving mechanism and dynamic system of migration between different pressure ststem changed regularly among space and time. A positive correlation between oil saturation of sandstone and quality of reservoir physical properties in middle part of the sag. Buoyancy is the dynamic controlling petroleum migration and accumulation in normal pressure zone, and the role of dynamic is unimportant for large-scale oil accumulation in deep overpressure area. The abnormally high pressures of shahejie formation in bonan sag increase oil saturation in sandstone, and also reduce lower limit of physical-property, which are favorable to hydrocarbons accumulation in overpressured area. The stratigraphic traps with hydrocarbon accumulate in the under layer of gypsum beds in upper part of es4s are mainly overpressured.