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Yang X.-Y.,Institute of Sedimentary Geology | Yang X.-Y.,Southwest Petroleum University | Bao H.-P.,Petrochina | Ren J.-F.,Petrochina | Ma Z.-R.,Petrochina
Natural Gas Geoscience | Year: 2015

The types of dolomites from the Ma 55 sub-member of Ordovician Majiagou Formation in Ordos Basin are micrite, siltcrystal-micrite and siltcrystal dolostones based on petrological analysis and geochemical methods. The results show that micrite dolostone is formed in ungated-restricted environment with high salinity by penecontemporaneous dolomitization, with micrite texture, garnet light in cathoduluinescence observation, lower degree of order. The average value of δ13C, δ18O and 87Sr/86Sr are -1.23‰, -7.59‰ and 0.70981, respectively. Siltcrystal-micrite dolostone is formed by many dolomitization for penecontemporaneous dolomitization of the micritic dolomite and reflux seepage dolomitization of the siltrystal dolomite in which are many lime-mud or dolomite-mud, whit siltcrystal-fine micrite texture, dirtier, no or extreme dark light in cathoduluinescence, 0.75 of degree of order. The average value of δ13C, δ18O and 87Sr/86Sr are -0.43‰, -7.29‰ and 0.70936, respectively. Siltcrystal dolostone shows the same as siltcrystal-micrite dolostone in cathoduluinescence, but the degree of order is 0.81. The average value of δ13C, δ18O and 87Sr/86Sr are -0.65‰,-6.46‰ and 0.70967, respectively. These indicate that siltcrystal dolostone is due to buried dolomitization with varying degrees of superimposed dolomitization. ©, 2015, Science Press. All right reserved. Source

Meng W.B.,State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation | Meng W.B.,Institute of Sedimentary Geology | Lu Z.X.,Sinopec | Liu J.D.,State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation | And 2 more authors.
Acta Petrologica Sinica | Year: 2011

The Middle Jurassic in western Sichuan is typically a red clastic bed without hydrocarbon generation capacity. Composed by four sandstones of Js 2 1 , Js2 2, Js2 3 and Js2 4 from top to bottom, the reservoir of the Upper Shaximiao Formation in Xingchang gas field is tight sandstone reservoir with average porosity of 9. 6% and average permeability of 0. 177 ×10 μm . The source rock of the gas pool is 1000 ∼ 3000m underlain Xujiahe Formation, which makes the Upper Shaximiao Formation a distal gas pool. Secondary porosity that was created dominantly by the dissolution of feldspar grains was markedly developed in the reservoir sandstone and has gready enhanced the tight sandstone reservoir property. Secondary porosity made much higher contribution to reservoir than primary porosity. It accounts for proximately 60% of thin-section point counted porosity. Several lines of evidence from bitumen occurrence and dissolved minerals of the sandstone, the distribution of kaolinite, the composition of formation water, and component of hydrocarbon inclusions in authigenic minerals suggest that the second porosity of the Upper Shaximiao Formation tight sandstone was created by the dissolution of organic acid from three sources. The first source of organic acid is from the underlain Xujiahe Formation consists of very thick organic-rich coal beds with III-type kerogen that expelled organic acid during hydrocarbon generation which was transported upwards through faults and fractures. The second source is a result of reaction of oxidant in the reservoir sandstone with hydrocarbon transported from deep beds, and the less important third source comes from dehydration process of the mudrock of the Shaximiao Formation. The first two sources of organic acid are responsible for most of the secondary porosity formation, while those created by organic acid from the third source are trivial due to their very small amount and failure in preservation. Regional water potential difference in the fifth member of the Xujiahe Formation during Jurassic and faults and fractures provide the conditions needed for organic acid transportation from the underlain Xujiahe Formation up into the Shaximiao Formation. The content of soluble components of the sandstone, paleotectonic location of the reservoir, and the degree of fault and fracture development and their distribution are major controlling factors for the forming and distribution of dissolved porosities. Source

Cheng L.,Institute of Sedimentary Geology | Cheng L.,State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation | Cheng L.,Sinopec | Wang Y.,Institute of Sedimentary Geology | And 5 more authors.
Acta Petrologica Sinica | Year: 2013

Black shales within the Sinian (Ediacaran) to Early Paleozoic strata from Upper Yangtze region are the three high quality source rocks of Sinian to Early Paleozoic marine Lower Assemblage in South China, which are generally thought to be the result of deposition under stagnant and suboxic conditions. According to the information of sedimentary structure and biological, ecological patterns, the sedimentary environment and burial condition of these black shales are discussed. The benthonic organisms and abundant natant organisms indicate that there were, in the sedimentary environment of black shales, an oxic upper-water in where abundant organisms lived and repropagated to provide organic-rich matters for deposits, and a suboxic or dysoxic bottom-water in where the decomposition of organic-matters were weakened and/or baffled for the preservation of organic-rich matters. In addition, the sedimentary structures formed by water-energy (e. g. the ranking orientation of fossils and their fragments, the oblique bedding in nonblack-shale rocks intercalated in black shales) show that there were measure of water-energy or an periodic and intermittent hydroenergy in the bottom-water. The water-energy was able to promote the oxygenic diffusion in bottom-water for the benthonic organisms. With the increase of the water-rich muddy deposits, the oxygen-exchanging efficiency between the deposited water and sea-water was weakening to intensify the stagnation and reduction of the deposited water inner deposits. Simultaneously, the organic-rich matter (e. g. organismic corpses and excreta) buried by deposits were decaying and decomposing to advance the reduction around them, so that the organic-rich matter were preserved to form the black shales. Source

Zhao J.X.,State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation | Zhao J.X.,Institute of Sedimentary Geology | Shen Z.J.,State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation | Shen Z.J.,Institute of Sedimentary Geology | And 7 more authors.
Acta Petrologica Sinica | Year: 2011

The development of sequence stratigraphy in recent years provides new approach for us to perform basin analysis. We divided the Yanchang Formation of Late Triassic in Ordos basin into 4 long-term and 22 medium-term base level cycles using sedimentology and high-resolution sequence stratigraphy methods. In Yanchang Stage, the deposit-sequence-structure evolution of Ordos basin can be summarized as the following; the structure subsided initially in LSCl and lakes were formed; the lake basins downwarped quickly in LSC2 and was most active in the late period; in LSC3 the circumjacent structures tended to be stable, with basin filling commenced and the lakes became shallow; in LSC4 fluvial deposition increased, and the basin turned into extracompensation stage, with the lakes silted and turned into residual lacustrine deposit This paper also put forward and defined the concept of sequence structure dynamics, which is considered as a subject that researches on the control and effect of tectonic movement, sea level fluctuation, source supply, climate and sedimentation on sequence structure development and its distribution laws during sequence filling process. Based on the comprehensive division of the Yanchang Formation sequence stratrigraphy, the sequence structure was classified into 5 types (A, Bl, B2, B3 and C) according to the sequence structure variation features formed in a datum variation cycle. Then we detailedly analyzed the sequence filling structure features, structure types, sequence filling dynamics process, developing types of sequence structure and filling process with respect of various locations and deposit system units of the Yanchang Formation, Ordos basin, and discussed the sequence structure dynamics characteristics of large continental depressed lake basins. Then we brought forward the "sequence structure distribution laws" for the first time, and constructed the sequence structure distribution pattern of large continental depressed lake basins of Yangchang formation, Ordos basin. It was considered that without important structural event or paroxysmal deposit event, only the type of adjacent sequence structures may distributed with superimposition. Source

Su Z.,State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation | Su Z.,Institute of Sedimentary Geology | Chen H.,State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation | Chen H.,Institute of Sedimentary Geology | And 6 more authors.
Acta Petrologica Sinica | Year: 2011

The types and genetic mechanism of the Majiagou dolomite in Ordos basin are investigated using petrological analysis and geochemical methods. The results show that dolostones in Majigou Formation mainly consist of dolomicrite and crystalline dolostone. Dolomicrite often associates with gypsum or halite, with medium jacinth light in cathoduluinescence observation, lower degree of order, and high content of Sr and Na It provides similar δ 13C and REE distribution pattern with micrite. The value of Sr isotope is close to that of sea water in the same period, and fluid-inclusion homogenization tempetrature approximate to earths surface. These suggest that dolomitization fluid is high salinity fluid that closely related with sea water, considered as penecontemporaneous dolomitization which can be explained by evaporation pump mode. Crystalline dolostone contains little associated minerals, only pyrite observed occasionally. This crystal structure is usually xenomorphic curve. Crystalline dolostone shows medium-weak jacinth light in cathoduluinescence, whith high degree of order and low content of Sr, while Na and Mn content is close to that of dolomicrite. REE distribution pattern is similar with dolomicrite and close to micrite. The distribution of δ 13C is also corresponding to micrite, and fluidinclusion homogenization tempetrature is hight. These indicate that dolomitization fluid of crystalline dolostone is similar with that of dolomicrite. Crystalline dolostone is buried dolomitisation, where continuous compaction from overlying deposits urges the sealed brine to flow laterally, thus supplies unfailing Me 2+ for dolomitisation. Source

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