| Bozhong-Liaodongwan Depression is one large-area part of Bohai Bay Basin covered by modern Bohai Sea. There are multiple source-reservoir-caprock assemblages in this area, demonstrating great abundance of petroleum resources. Overpressures are encountered in the exploration and drilling processes. The study of the characteristics, prediction, origin, and distribution of overpressure can serve as scientific basis for reservoir prediction, migration and reservoiring analysis, and drilling hazard prevention. In this thesis, the author carry out through research on the characteristics, prediction, origin, distribution, and their relationship with petroleum accumulations of overpressures in Bozhong-Liaodongwan Depression, in the guidance of the theories and methods of petroleum geology, geophysical exploration, and petroleum reservoiring, etc. The geology background data, drilling data, geophysical data and laboratory testing results are used in this study and the previous research achievements are absorbed. The research works in this thesis can be divided into four ajor parts: ①The processing and analysis of measured pressure data, and the summarization of the characteristics of measured pressure data. ②The analysis of the corresponding characteristics of geophysical data to overpressure using well logs and seismic velocities; the 1-dimentional,2-dimentional, and 3-dimentional overpressure prediction based on the corresponding characteristics; the 3-dimentioanl visualization and distribution maps of overpressures in study area. ③The investigation of overpressure origins using geology data, well logs, testing results, etc.; the 1-dimentional,2-dimentional, and 3-dimentional simulations of the burial, thermal, and overpressure evolution histories.④The discussion of the sealing ability of the overpressure, and the preservation effect of primary porosity in deep-buried overpressured reservoir. These works lead to the following conclusions:1.1430 points of drill stem test (DST) measured pressure data of 714 wells reveal that the overpressures in study area mainly occur in the Paleocene Dongying Formation (Ed) and Shahejie Formation (Es), and horizontally distribute in the sags and the neighboring uplifts. In Bozhong Depression, there is no measured overpressure in Neocene Guantao Formation (Ng) and Minghuazhen Formation (Nm); the middle and lower Ed Formation (Ed2 and Ed3) are featured by significant overpressure, which distribute between the burial depth of 2725 m to 3995 m and reaches the maximum pressure of 60 MPa at the depth of 3650 m, with the maximum overpressure coefficient at 1.78; overpressures in Es Formation mainly develop in the first and third member (Es1 and Es3), occurring below the depth of 2500 m, and the maximum measured overpressure coefficient is 1.78. In Liaodongwan Depression, the Ng and Nm Formations are generally normal pressure; the measured overpressure in Ed Formation mainly develop in Ed2 and Ed3, which distributes between the depth of 2112 m to 3435 m and reaches the maximum pressure of 68 MPa at the depth of 3435 m, with the maximum overpressure coefficient at 1.92; measured overpressures in Es Formation are seen between the depth of 1825 m and 3825 m, and the overpressure coefficient ranges 1.2-1.78.2. The acoustic, resistivity and density logs of 116 wells in study area show good corresponding characteristics to overpressure, among which the acoustic log show more obvious correspondence. The acoustic travel time in overpressured shale is much different from which in normal-pressure shale, featured by the larger travel time than the normal compaction trend. The seismic velocity near the borehole show good corresponding characteristics to overpressure as well as the acoustic log, with the velocity lower than normal compaction trend. Thus the corresponding characteristics of acoustic log in shale and the seismic velocity to overpressure could be employed to establish overpressure prediction models both 1-dimentional and 2-dimentional, and then the general characteristics and distribution patterns of overpressure in Bozhong-Liaodongwan Depression can be estimated.3. The 1-dimentional and 2-dimentional pressure prediction validated by the measured pressure data draws following conclusions on the pore pressure develop conditions and distribution patterns: ①The top surface of overpressure in Bozhong Depression ranges 2000-4500 m, shallower in the sag margins and neighboring uplifts and deeper in the sag centers; the overpressure mainly develops in Ed and Es3, with the magnitude of 12～30 MPa; overpressures in Es1 and Es2 are relatively smaller in magnitude. The overpressure in Bozhong Depression can be divided into two systems by the mildly overpressured Es1 and Es2, above is the Ed overpressure system and below is the Es (Es3)overpressure system. ②Horizontally, the Ed overpressure in Bozhong Depression mainly distributes in the center part of Bozhong Sag with the maximum pressure coefficient at 1.8 and decreases to the surrounding fields, and small overpressure centers can be found in the Bodong Sag and Huanghekou Sag with the maximum pressure coefficient around 1.5; the Es overpressure demonstrates a wide-spread circle distribution with the maximum pressure coefficient in central Bozhong Sag over 1.8. ③The top surface of overpressure in Liaodongwan Depression ranges 1000-3500 m, with the shallowest depth of 1000 m in the uplift close to the north end of Liaoxi Sag, and the deepest in the central part of Liaozhong Sag; the overpressure mainly develops in Ed2 and Es Formation, with the magnitude of 8-20 MPa; overpressures in Ed3 and Es2 are relatively smaller in overpressure magnitude. The overpressure in Liaodongwan Depression can also be divided into two systems: the Ed overpressure system featured by the overpressures in the huge-thick shales of Ed2 and Es overpressure system. ④Horizontally, the Ed overpressure in northern Liaodongwan Depression forms a significantly overpressured zone along the trend of the sags, with the maximum pressure coefficient at 1.8; two overpressure belts form in the central Liaoxi Sag and central-southern Liaozhong Sag, with the maximum pressure coefficient around 1.5; The Es overpressure form a severe overpressured zone in the north end of the depression with the maximum pressure coefficient reaching 1.9, and distributes in two wide belts in the central-southern area of Liaoxi Sag and Liaozhong Sag with the maximum pressure coefficient around 1.8.4. Through the comprehensive analysis of the sedimentation/subduction rates, measured acoustic velocity-vertical effective stress relationship, sampled acoustic velocity-density relationship, and source rock thermal maturation history, the thesis propose the Ed overpressure in Bozhong Depression was originated by the disequilibrium compaction at the fast sedimentation rate, and the hydrocarbon generation in the deep buried (current depth >3500 m) lower Ed from 10 Ma ago may have made additional contributions to the current overpressure situation; the Es overpressure was also originated by disequilibrium compaction, and the hydrocarbon generation of Es currently deeper than 4000 m from 14 Ma ago have made major contributions to the current overpressure magnitude.5. The comprehensive analysis of the sedimentation/subduction rates, measured acoustic velocity-vertical effective stress relationship, sampled acoustic velocity-density relationship, and source rock thermal maturation history suggest the Ed overpressure (mostly Ed2 overpressure) in Liaodongwan Depression was originated by the disequilibrium compaction at the fast sedimentation rate, and the hydrocarbon generation in the deep buried (depth >3000 m) lower Ed may have made additional contributions to the current overpressure situation; the Es overpressure was also originated by disequilibrium compaction, and the hydrocarbon generation of Es4 and Es3 have added to the magnitude of overpressure.6. The evolution simulation of the typical sections in Bozhong Depression shows the disequilibrium compaction caused overpressure in Es3 was formed in the fine-grain sediments at the sedimentation rate over 450 m/Ma. The lacustrine source rocks of Es3 matured 36 Ma ago when the burial depth over 2000 m and Ro >0.7%, and its generation behavior caused additional overpressure in Es and transferred the overpressure to the reservoir rocks in Es1 and Es2. Then the regional uplift and erosion movement at the end of Es1 caused the leakage and decrease of overpressure in Es. The fast deposition of Ed lacustrine strata caused the disequilibrium compaction and overpressure, in the meanwhile the continuous burial of Es kept the hydrocarbon generation and added to the overpressure in Es Formation. The regional uplift and erosion movement at the end of Ng caused the leakage and decrease of overpressure in Ed, while the deposition of Nm and Np afterwards has made the Paleocene buried deeper and the hydrocarbon generation in Ed and Es has added significantly the magnitude of overpressure in the source rocks and transferred to the reservoirs. The hydrocarbon generation in Ed and Es since 14 Ma ago and their transfer effect has formed the widely distributed overpressure in Bozhong Depression.7. In Liaodongwan Depression, the simulated overpressure evolution history of the northern part is different from that of the central-southern area due to the difference their sedimentary characteristics. In the northern part of Liaodongwan Depression, two overpressure centers can be observed in Liaoxi Sag and Liaozhong Sag, between which the overpressure magnitude in Liaozhong Sag is larger than that in Liaoxi Sag. The overpressures in both sags has been originated the shaped by the disequilibrium compaction and hydrocarbon generation of Es3 and Ed. The Ed overpressure in Liaoxi sag is dominantly caused by the disequilibrium compaction as the buried depth is too shallow for the Ed source rocks mature, while the hydrocarbon generation of Ed3 since 10 Ma ago has shaped the current overpressure conditions in Liaozhong Sag. In the central-southern area of Liaodongwan Depression, the overpressures in Paleocene was initially generated by the disequilibrium compaction of Es3, and the hydrocarbon generation of Es after it thermally matured raised the overpressure magnitude. The thickness of Ed2 in this area is smaller than that in the northern part, thus the disequilibrium of Ed2 is not significant. As the buried depth get deeper and deeper since the deposition of Ng, the maturity of source rocks in Ed and Es get higher, and the hydrocarbon generation in both formations contribute more to the overall overpressure magnitude up to date.8. Large-scale overpressure zones are developed in the moderate-shallow buried strata (depth 1000～3000 m) in northern Liaodongwan Depression. The overpressure bearing layers are dominantly semi-deep and deep lacustrine shales of Ed Formation. Statistically, the total thickness of the shales takes up to 90% of the strata thickness. The thickness of overpressure bearing shales in different places ranges 276-952 m, and the maximum thickness of the single layers reaches hundreds of meters. The overpressure magnitude in these shales ranges 2.4-17.3 MPa. The proved oil and gas layers in this area are mostly under the overpressured shales, suggesting the excellent sealing effect of the shallowly buried overpressures shales blocks the upward leakage or fault-along transferring of oil and gas.9. The porosity of sandstone reservoirs in Bozhong Depression generally decreases as the depth increasing, but measured off-trend high porosities exist under the depth of 3000 m. The porosity on normal compaction trend ranges 12～23% around the depth of 3000 m, while the off-trend porosity at the same depth can be as high as 33%; the normal compaction trend porosity ranges 10～20% around the depth of 3500 m, while the off-trend porosity at the same depth can be as high as 30%; the measured porosity in the deep reservoir below 4000 m can reach 28%, deviating significantly from the normal compaction trend. Most of the off～trend porosities are found in the overpressured layers. The pores in the overpressured layers are mainly primary inter-granule pores and dissolution pores are not developed according to the observation of pore characteristics. The measured sandstone porosities show positive relationship with the overpressure magnitude, and the vertical effective stress analysis suggest that given the same vertical effective stress, the porosity in overpressured layers are generally greater than that in normal compaction layers. These analysis indicate the disequilibrium compaction caused overpressure has retarded the compaction, therefore preserved the primary porosity. It is then proposed that good reservoir with high porosity can be predicted in the deep overpressured layers under 3300 m in Bozhong Depression. |
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