The Laizhou Bay Sag, one of the oil-bearing sags with large exploration potential in the South Bohai Sea of China, contains two sets of high-quality hydrocarbon source rocks, i.e. Member 3 and Member 4 of Shahejie Fm (E 2 s 3 and E 2 s 4 ). As an important hydrocarbon accumulation zone in the sag, the KL16-1 low bulge is found to have oil reservoirs in the Neogene Guantao Fm (N 1 g), the Paleogene Dongying Fm (E 3 d), the Paleogene Shahejie Fm (E 2 s), and the Mesozoic buried hill formation. It is characterized by distinct features of composite reservoirs. Nonetheless, its hydrocarbon accumulation process and mechanism are unknown yet. In this paper, the hydrocarbon generation modelling of source rocks, combined with the characteristics of oil-source rock biomarker compounds and fluid inclusions, is used to restore the hydrocarbon accumulation process in the KL16-1 low bulge. Crude oil in all three KL16-1 plays is characterized by low Pr/Ph, low gammacerane, high 4-methyl sterane and high dinosterane. In other words, it is oil of low maturity to maturity. As the biomarker index of the oil is very similar to that of the E 2 s 3 source rocks, it is thought that E 2 s 3 is the primary source rock layer, while E 2 s 4 is the secondary source rock layer. In addition, episodic oil and gas charging happened in two phases under high pressure in the late period (from 5.0 Ma till now).
Abstract In the JZ27-A well block of the strike-slip and compression-shear booster zone in the JZ27 section of the Liaodong Bay in the Bohai Sea area of the Tan-Lu Fault, overpressure developed in the third member of the Shahejie Formation, and the pressure coefficient reached an average of 1.5. Thick mudstones sealed the overpressure on the top. Particle fragmentation was obvious in the overpressure zone. The area and thickness of the overpressure were controlled by the range of the pressure booster zone and the intense development of strike-slip and compression-shear zone, mainly restricted to the third member of the Shahejie Formation in the JZ27-A well block. The origin for overpressure was interpreted to be as follows. From the end of the third member of the Dongying Formation to the early stage of the second member of the Dongying Formation, influenced by the intense dextral strike-slip of the Tan-Lu Fault, stress concentrated in the JZ27-A well block of the strike-slip and compression-shear booster zone. Thereby, horizontal structural compression stress formed. At the same time, because of overlying thick-bedded mudstones, stress could not be released effectively. As a result, overpressure occurred. According to the balance of reservoir pressure, the contribution of strike-slip stress to overpressure was quantitatively evaluated. The contribution of strike - slip stress to overpressure was 30% - 35% and occurred earlier than the period with large amounts of hydrocarbon generation and expulsion. The overpressure impeded the charging of oil tremendously.
The BZ19-6 deep buried-hill structural belt in the southwest of Bozhong Sag, Bohai Bay Basin, is a newly discovered super-giant oil and gas bearing area. The study on its reservoirs is still in the early stage, and the characteristics and control factors of reservoir development are not understood deeply. In this paper, cores, sidewall cores, rock sections were analyzed and described. Then, based on regional structural setting, mud logging and logging data, the buried-hill reservoirs in this area were analyzed from the aspects of petrological characteristics, reservoir space types and physical properties, the inherent factors influencing the development of the reservoirs were discussed, and distribution laws of the reservoirs were investigated. And the following research results were obtained. First, the deep buried-hill reservoirs of this belt are a pan-buried hill reservoir system composed of the Palaeocene–Eocene Kongdian Fm glutenite in the upper part and the Archean buried-hill metamorphic granite in the lower part. A multi-layer reservoir structure of glutenite pore zone, weathering crust dissolution fracture zone and inner fracture zone is formed. These reservoirs are complex in genesis and diverse in type. Second, the Archean buried-hill metamorphic granite reservoir can be vertically divided into weathering crust, inner fracture zone and tight zone, and it presents the dual characteristics of porous and fractured media. Third, the buried-hill weathering crust is mainly affected by strong dissolution and leaching superimposed with fracturing, forming fractured-porous reservoir space. The reservoir of inner fracture zone is mainly controlled by the superimposition of three-phrase fractures, which forms the main development period of buried-hill fractures since the Yanshanian. Fourth, the glutenite of Kongdian Fm is a typical sieve deposit and it is mainly controlled by the late dissolution. Fifth, migmatization and supercritical fluid cryptoexplosion play a constructive role in the development of the reservoirs. In conclusion, the understanding of buried-hill glutenite and metamorphic reservoir system developed in this belt is conductive to determining the target and direction of next oil and gas exploration in this area.
Based on the study of the abnormal pressure of mudstone and reservoirs of south Liaozhong sag, Bohai Bay Basin, the structure of the abnormal pressure compartment in the Paleogene of south Liaozhong sag and its impact on oil and gas migration have been analyzed. The pressure compartment in the study area has a “sandwich structure”, with the mudstone of Paleogene Dongying Formation as cap rock, the reservoirs of the upper Shahejie Formation of the Paleogene as the transport interlayers, and the mudstone from the lower Shahejie Formation as the hydrocarbon source rock. Based on overpressure formation mechanism and basin-modeling, the pre-neotectonic movement pressure of the transport interlayers was reconstructed, then the rupture of the pressure compartment and pressure differentiation caused by Tanlu fault zone during the neo-tectonic movement were investigated. The hydrocarbon migration was simulated under different pressure conditions before and after the neo-tectonic movement, to find out the relationship between the neo-tectonic movement of Tanlu fault, abnormal pressure and oil and gas migration. The simulation results show that the differential activity within Tanlu fault zone caused the differentiation of pressure and increase of fluid potential gradient; and the strike-slip pull-apart area with low potential caused by pump-sucking effect became the main direction of hydrocarbon migration in Tanlu fault zone. The comprehensive research of the late-stage hydrocarbon accumulation shows that, there are inner and outer compartment hydrocarbon accumulation systems in the south Liaodong sag, and the transport interlayer in the inner compartment is the “hydrocarbon transfer station” of the shallow outer compartment, and determines the hydrocarbon plane distribution.
The intact volcanic edifice had been formed and finalized during the depositional stage of Mesozoic Yixian Formation in the Bohai Sea. The residual body of the intact volcanic edifice after reformation was defined as the basement reformed volcanic edifice. Based on drilling, seismic, well logging and microscopic data in the study area, characteristics of the basement reformed volcanic edifice were well discussed, and its control on oil and gas was also investigated. Results showed that the basement reformed volcanic edifice was jointly controlled by magmatic property, paleo-geomorphology and late reformation, and the late fault cutting and differential denudation were major reformation ways of the volcanic edifice; strong and weak volcanic eruption patterns developed in the study belonged to the inverted-sequence eruption, and the explosive facies and the intermediate-acidic or acidic effusive facies were the favorable reservoir facies; weathering, cycles and period boundaries controlled vertical distribution of high-quality lithofacies, and coupling between each periodic interface at different levels and residual high-quality lithofacies was the key to oil and gas enrichment. Major exploration targets of volcanic rocks were to find periodic interfaces in the volcanic edifice with the residual high-quality lithofacies within 150 m from the weathering crust or eruption surface.
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