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: 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: October 2017 Source: Petroleum Exploration and Development, Volume 44, Issue 5 Author(s): Wenzhi ZHAO, Guoqi WEI, Wei YANG, Wuling MO, Wuren XIE, Nan SU, Mancang LIU, Fuying ZENG, Saijun WU Abstract New understandings of the geology of Sichuan Basin were achieved in the progress of natural gas exploration in the Sinian-Cambrian strata in Sichuan Basin. An NE trending intracratonic rift was found in the Wanyuan-Dazhou area, northeastern Sichuan Basin. Based on seismic data interpretation, outcrop data and regional tectonic background analysis, we studied the boundary, distribution, formation and evolution history of Wanyuan-Dazhou rift. The following findings were obtained: (1) The seismic section indicates that a steep-slope belt indicating platform margin facies was developed during the deposition of the first and second Member (Z 2 dn 1 -Z 2 dn 2 ) of the Sinian Dengying Formation. The rift generally strikes NE. (2) The thicknesses of the first and second Member of the Dengying Formation are thicker than the third and fourth Member (Z 2 dn 3 -Z 2 dn 4 ) at the periphery of the rift and vice versa inside the rift. (3) The rift formed during the deposition of Z 2 dn 1 -Z 2 dn 2 in the Sinian. Filling and subsidence occurred during the deposition of Z 2 dn 3 -Z 2 dn 4 in the Sinian. The shrinkage of the rift happened during the deposition of the Lower Cambrian Maidiping Formation – Qiongzhusi Formation. Regional extension and uplift in the Nanhua controlled rift formation. The discovery of the Wanyuan-Dazhou rift changed the traditional understanding of the tectonic and sedimentary framework of Sichuan Basin in the Sinian and Early Cambrian. The periphery of the rift has significant potential for natural gas exploration due to its superior natural gas accumulation conditions in the Sinian.

Description: Publication date: October 2017 Source: Petroleum Exploration and Development, Volume 44, Issue 5 Author(s): Yanqun QIN, Guangya ZHANG, Zhifeng JI, Zhi LI, Yiping WU, Xinglong WANG, Xu LIANG Abstract Based on the tectonic evolution, sedimentary filling and hydrocarbon geological characteristics, this paper establishes a variety of deep water fan models and accumulation patterns and analyzes exploration potential of deep water area in East Indian basins. Due to the breakup of East Gondwana, the passive continental margins of East India are evolved. The basins in these margins experienced four evolution stages, which include intracratonic rift (P 1 -T), East Gondwana rift (J 1 -K 1 ), post-rifted thermal subsidence (K 2 ), and passive continental margin (K 3 -N). There are three obvious unconformities which were formed in the end stages of post-rift thermal subsidence and first and second phases of passive continental margin. Four sets of structural layers were formed, rift stage, and the first, second and third stages of passive continental marginal structural formation. The main deposition of these basins is the sediments evolved during the passive continental margin stage. Two prolific fan systems were developed: superposition fans are complicated in near shore and delta; low fans are located at fault slope-break while turbidity fans are located in front of delta. Four sets of proven source rocks were developed, the Lower Permian, Lower Cretaceous, Upper Cretaceous and Paleogene. There are many middle porosity reservoirs and lowstand permeability reservoirs, such as sandstone and local carbonate in rift period, sandstone of delta and deep water gravity flow in the rifted thermal subsidence and passive continental margin stages. Many seals were developed in the basins, regional seals in drifted stage and local seals in other stages, and the seals thickness change are enormous. There are eight plays in these basins which include Upper Cretaceous sandstone, Paleogene sandstone, Neogene sandstone, etc. The hydrocarbon exploration of deep water area during the passive continental margin stage should focus on Krishna and Cauvery river delta on the plane, and delta sandstone related to fault, lowstand fan located at fault slope-break and near shore fan complex in vicinity of 85°E ridge and south offshore of Cauvery basin vertically.

Description: Publication date: October 2017 Source: Petroleum Exploration and Development, Volume 44, Issue 5 Author(s): Yunfeng ZHANG, Fei TAN, Haizhou QU, Zhiqi ZHONG, Yong LIU, Xinsheng LUO, Zhenyu WANG, Fang QU Abstract Based on the fine characterization of weathering crust paleokarst landform of the Ordovician Yingshan Formation in Lungu area, Tarim Basin, the size, number and reservoir controlling characteristics of karst monadnocks have been investigated quantitatively. Impression method and karst monadnock volume formulas were employed to characterize the various levels of karst microtopography, to work out the number, relative relief, area, volume and other parameters of monadnocks, and analyze the reservoir development situation at different parts of monadnock. The results showed that the weathering crust karst reservoirs in Lungu area are 140-160 m in relative altitude, and thinning from karst highland to karst slope and terrace. Two hundred fifty-two karst monadnocks have been identified in the study area. Among them, type I and type II monadnocks are fewer in number, but larger in bulk volume, and are favorable reservoir development zone. The karst monadnocks have a high coincidence ratio with current structural high points of 96.42%, and the monadnock morphology has remained basically unchanged during the late tectonic movement, thus “karstification-reservoir-accumulation” configuration has been kept good. In the main part (core) of karst monadnocks, there are cave, vug, fracture-vug, and fracture reservoirs, thicker high quality reservoir, and all reservoir parameters are better than those of karst monadnock wings. It is found that the relative amplitude of karst monadnock is positively correlated with single well productivity, and the scale and position of monadnock are closely related with productivity.