Description: Publication date: October 2017 Source: Petroleum Exploration and Development, Volume 44, Issue 5 Author(s): Wei ZHANG, Jinqiang LIANG, Jing'an LU, Jiangong WEI, Pibo SU, Yunxin FANG, Yiqun GUO, Shengxiong YANG, Guangxue ZHANG Abstract Based on the comprehensive interpretation of cores, loggings and 2D/3D seismic data of Shenhu GMGS3 drilling area in the northern South China Sea, the distribution characteristics, differential accumulation mechanism and reservoir forming mechanism of diffusion type natural gas hydrate with high saturation discovered from clayey silt reservoirs were investigated. The following findings are reached through the research: (1) Gas hydrate with high saturation often displays high resistivity, low interval transit time, and strong bottom-simulating reflectors (BSRs), and accompanies with fluid seepage phenomena beneath BSRs, such as mud diapiric structure and gas chimney. (2) The gas hydrate reservoirs are dominated by fine grained clayey silt sediments, and the reservoirs have higher porosity and permeability in local parts. (3) The gas hydrate is largely type I, whereas type II gas hydrate may exist below the type I gas hydrate. (4) The gas sources are mixed microbial and thermogenic gases, and the thermogenic gas originated from the deep formation in the center of Baiyun Sag migrated into shallow strata through faults, mud diapirs and gas chimneys, then was mixed with microbial gas in situ and continued to migrate until they accumulated in the temperature and pressure stability zone and formed diffusion type gas hydrate with high saturation finally. (5) The fluid migration system influenced and controlled the differential distribution of gas hydrate with high saturation.

Description: Publication date: December 2017 Source: Petroleum Exploration and Development, Volume 44, Issue 6 Author(s): Xinjian ZHU, Jianfa CHEN, Jianjun WU, Yifan WANG, Baoshou ZHANG, Ke ZHANG, Liwen HE Based on the carbon isotopic compositions of Cambrian-Ordovician source rocks Kerogen Samples and Paleozoic crude oil in the platform region of Tarim Basin, the origin and source of Paleozoic crude oil were investigated. There are at least two sets of source rocks with different carbon isotope compositions in the Cambrian, the Lower Cambrian source rock with lighter carbon isotope composition and Middle-Upper Cambrian source rock with heavier carbon isotope composition, while the Ordovician source rock is somewhere in between. The δ 13 C values of Paleozoic crude oil samples are wide in distribution range, from −35.2‰ to −28.1‰. The crude oil with lighter carbon isotopic compositions ( δ 13 C〈−34.0‰) was mainly derived from Lower Cambrian source rock, and the crude oil with heavier carbon isotopic composition ( δ 13 C>−29.0‰) was mainly derived from the Middle-Upper Cambrian source rocks, and the crude oil with δ 13 C value in between may be derived from Cambrian source rocks. It is concluded through analysis that the Cambrian source rock could become the major source rock in the Tarim Basin and the platform region has huge potential oil and gas resources in the deep formations.

Description: Publication date: October 2017 Source: Petroleum Exploration and Development, Volume 44, Issue 5 Author(s): Li ZHANG, Zhidong BAO, Yanbo LIN, Yuming CHEN, Xiaohai LIN, Luxing DOU, Bin KONG Abstract Based on outcrop, core, logging and mud logging data, and modern deposition analog, the sandbodies genetic types and spatial distribution in different facies belts of the shallow water delta in the first Member of Cretaceous Yaojia Formation (K 2 y 1 ) of Qian'an area in Changling sag of southern Songliao Basin were analyzed, and the sedimentary model of shallow water delta was established according to deposition process. Active channel sandbodies, point bar sandbodies and bifurcation bar sandbodies in distributary channels and complex sandbodies formed the main sandbodies in the delta plain, while terminal distributary channels and mouth bar sandbodies formed the main sandbodies in the delta front. The delta prograded gradually under the construction of river mouth and reconstruction of river erosion, the early delta front evolved into delta plain and was reformed by distributary channels, and new delta fronts were formed at the river mouth ahead constantly. Under the control of the above deposition process, the size of main distributary channels in delta plain-delta front declined, while mouth bars developed increasingly. The sandbodies of different genetic types formed framework of river-dominated shallow water delta. The point bar and active channel sandbodies in main distributary channels of the delta, with the best reservoir quality, are lithologic reservoir targets in future exploration.

Description: Publication date: October 2017 Source: Petroleum Exploration and Development, Volume 44, Issue 5 Author(s): Jie CHI, Binshan JU, Guangzhong LYU, Xing ZHANG, Jiabei WANG Abstract Based on the theory of non-Darcy seepage, a mathematical model for CO 2 miscible and immiscible concurrent flooding considering changes of oil viscosity and threshold pressure gradient of oil and CO 2 is established. A computational method of critical well spacing of CO 2 miscible and immiscible concurrent flooding in ultra-low permeability reservoirs is deduced by solving the distribution of CO 2 concentration in miscible flooding area with the mass transfer-diffusion-absorption equation and solving the saturation equation of immiscible affected area with characteristic line method. A critical well spacing example is built in the F142 and G89 reservoir blocks and the results show: (1) The critical well spacing increases with gas injection pressure, while decreases with gas injection speed; (2) The contribution of length in pure CO 2 seepage area to the critical well spacing is the largest, the contribution of length in CO 2 -Oil effective mass transfer area and immiscible affected area is secondary, the contribution of pure oil area is the least, the gap of length between pure CO 2 seepage area and CO 2 -Oil effective mass transfer area and immiscible affected area decreases with gas injection speed and it increases with the decreasing of gas injection speed, meanwhile, the law is more significant; (3) Pressure drop gradient of miscible affected area is significantly different from that of immiscible affected area and pressure drop gradient of CO 2 -Oil effective mass transfer area in miscible affected area is bigger than that of immiscible affected area.

Description: Publication date: December 2017 Source: Petroleum Exploration and Development, Volume 44, Issue 6 Author(s): Yu PENG, Jinzhou ZHAO, Yongming LI To simulate the evolution of wellbore creep accurately, predict and prevent severe accidents such as borehole wall sloughing, casing collapse and sticking of the drill, based on previous studies, the springpot element was introduced into the classical element model and the creep compliances of the fractional constitutive models were deduced. The good fitting effect of fractional constitutive model was verified. The study shows the fractional constitutive model can simulate creep with high accuracy and less input parameters, and the physical significance of the input parameters are clearer. According to the correspondence principle of viscoelastic theory, a wellbore creep model including drilling and killing processes was built up. By adjusting the value of fractional orders, the model can transform between the models of ideal elastic material and standard solid, which implies the classical wellbore shrinkage model based on standard solid model and ideal elastic model are just special cases of this model. If the fractional order is adjusted, the creep curve will change asymmetrically, which can be can be regulated by the speeding up of the transient creep and lowering the rate of steady creep, which can not be accomplished by adjusting one parameter in the classical models. The fractional constitutive model can fit complicated non-linear creep experiment data better than other models.

Description: Publication date: October 2017 Source: Petroleum Exploration and Development, Volume 44, Issue 5 Author(s): Qinhong HU, Yuxiang ZHANG, Xianghao MENG, Zheng LI, Zhonghuai XIE, Maowen LI Abstract For typical blocky, laminated and bedded mudrock samples from the Paleogene Shahejie Formation in the Dongying Sag of Bohai Bay Basin, this work systematically focuses on their structure characterization of multiple micro-nano pore networks. A use of mercury injection capillary pressure (MICP) documented the presence of multiple μm-nm pore networks, and obtained their respective porosity, permeability and tortuosity. Different sample sizes (500-841 μm GRI fractions, 1 cm-sized cubes, and 2.54 cm in diameter and 2-3 cm in height core plugs) and approaches (low-pressure N 2 gas physisorption, GRI matrix permeability, MICP, helium pycnometry, and pulse decay permeameter) were used to measure pore size distribution, porosity and permeability. The average porosity and matrix permeability determined from MICP are (6.31±1.64)% and (27.4±31.1)×10 -9 μm 2 , the pore throat diameter of pores is mainly around 5 nm, and the median pore throat diameter based on 50% of final cumulative volume is (8.20±3.01) nm in shale. The pore-throat ratios decrease with a decrease of pore size diameter. Moreover, the permeability of shale samples with lamination is nearly 20 times larger than matrix permeability. The geometrical tortuosity of the nano-scale 2.8−10.0 nm pore networks is 8.44 in these shales, which indicates a poor connectivity of matrix pore network and low flow capability. Overall, the variable and limited pore connectivity of shale samples will affect hydrocarbon preservation and recovery.

Description: Publication date: October 2017 Source: Petroleum Exploration and Development, Volume 44, Issue 5 Author(s): Yana CHEN, Anjiang SHEN, Liyin PAN, Jie ZHANG, Xiaofang WANG Abstract Based on the drilling cores and slice observations, single well data and geochemical analysis, this paper analyzed features, origin and distribution of the 4 th Member reservoirs of Sinian Dengying Formation (Z 2 dn 4 ) in the Sichuan Basin. It is demonstrated that the main reservoir is a set of microbial dolomites. The discovery of spherical dolomite has revealed that the dolomitization was related to the microbial action, belonging to the early protodolomite of low-temperature precipitation; the primary matrix pores and the penecontemporaneous eroded pores constituted the subject of the reservoir space, which was not due to the interlayer karst process related to the Tongwan Movement and burial-hydrothermal dissolutional process. The microbial mound-shoal complex and penecontemporaneous dissolution mainly control the development and distribution of the scaled reservoirs in Z 2 dn 4 . The microbial dolomite reservoir surrounding the intracratonic rift had a large thickness, good continuity and high quality, and was an important target of the survey.

Description: Publication date: October 2017 Source: Petroleum Exploration and Development, Volume 44, Issue 5 Author(s): Zhonghua LIU, Lianteng SONG, Changsheng WANG, Ting SUN, Xiaoming YANG, Xia LI Abstract For transverse isotropic fast formations, the evaluation method of the least horizontal principal stress by using logging data is an important unresolved issue. An innovative method is proposed to solve this problem by derivation of five independent stiffness coefficients ( C 11 , C 33 , C 44 , C 66 and C 13 ) in this kind of formation. Based on the functional relations between acoustic anisotropy coefficients and clay volume, and that between different stiffness coefficients, which are all approved by the assorted experiment data, an effective method is built to calculate the stiffness coefficients and the least horizontal stress of anisotropic fast formations. Successful applications in the Ordos Basin illustrate that the method is complementary to that based on the horizontal shear wave velocity which is only fit for slow formations, and is applicable to evaluating rock mechanical parameters of tight oil and gas reservoirs and selecting intervals for fracturing and testing oil.

Description: Publication date: February 2018 Source: Petroleum Exploration and Development, Volume 45, Issue 1 Author(s): Junqiao LIU, Haixue WANG, Yanfang LYU, Tongwen SUN, Mengdi ZHANG, Wei HE, Yonghe SUN, Tong ZHANG, Chao WANG, Lanzhu CAO The control effects of different occurrence faults on oil and gas accumulation and distribution in the outer slope area of oil and gas reservoirs were studied taking the south-central Wen'an slope of the Jizhong depression in the Bohai Bay Basin as an example. Based on 3D seismic data and the distribution of oil and water, the controlling differences between consequent fault and antithetic fault were analyzed and compared from the formation and evolution rule of faults and the formation mechanism of fault traps, including development positions of the consequent fault traps and antithetic fault traps, oil and gas distribution horizon adjusted by fault and formation period of fault traps. The differences between consequent faults and antithetic faults in controlling reservoirs have three main aspects: (1) Consequent fault traps and antithetic fault traps are in different positions, the consequent fault traps are at the segmented growing point in the hanging wall of “hard-linkage” faults, while the antithetic fault traps are developed in the position with the largest throw in the footwall because of tilting action; (2) The two kinds of faults result in different oil and gas distribution vertically, oil and gas adjusted by consequent faults is distributed in a single layer or multi-layers, while oil and gas adjusted by antithetic faults occur in single layers; (3) The two kinds of fault traps are formed in different periods, the consequent fault traps are formed at the time when the related faults enter the stage of “hard-linkage”, while the antithetic fault traps are formed at the beginning of the fault active period.

Description: Publication date: February 2018 Source: Petroleum Exploration and Development, Volume 45, Issue 1 Author(s): Hongquan KANG, Jinluo MENG, Tao CHENG, Huaicun JIA, Bo BAI, Minggang LI To make clear about the sedimentary facies types and distribution of deep water sandstone reservoirs in Campos basin of Brazil, this paper researches the characteristics of deep-water sedimentary system in Campos basin through the comprehensive analysis of drilling, logging and seismic data. There are 3 subfacies and 7 microfacies in the study area. There are 3 channels from south to north in Upper Cretaceous Maastrichtian, and the sedimentary incised valley and compound channels developed in micro-salt basin are the main deep water depositional types. The Paleocene to Eocene dominated by sedimentary incised valley and eroded compound channel deposits, also include 3 channel systems. From Oligocene to Miocene, the main deposition type is lobe, which is mainly distributed in central-north of the basin. Corresponding to deep water depositional stages, 3 kinds of depositional models are found. From Turonian to Maastrichtian of Upper Cretaceous, with tectonic uplift, strong source material supply, and the negative topography produced by salt rock movement providing favorable accommodation for sand deposition, the depositional model was terrigenous direct feed mechanism with sedimentary incised valley and compound channels in micro salt basin. From Paleocene to Eocene, as the amplitude of tectonic uplift reached the maximum and the accompanied erosion peaked, accommodation space offered by micro salt basin was leveled up; the depositional model was terrigenous direct feed mechanism with sedimentary valley and incised compound channels. From Oligocene to Miocene, because of sable tectonics, sea level fluctuation is the main controlling factor for deep water deposition, so the depositional model was wide shelf indirect feed mechanism with bypass incised valley and lobe. The analysis of the characteristics and controlling factors of the 3 types deep-water sedimentary systems during 3 different stages in Campos Basin can provide valuable reference for the oil exploration in deep-water deposits in the Campos Basin and across the world.