Based on field investigations and detailed analyses of lithofacies of the 18 representative sections from the various sedimentary settings,three second-order eustatic sea level changes and three sequence bottom boundaries are identified within the Ediacaran Doushantuo Formation of the Yangtze Plate,South China. The major sequence boundary(SB1)at the base of the Doushantuo Formation which is marked by the interface between the Cryogenian Nantuo diamictites and overlaying cap dolomite. The two major sequence boundaries(SB2 and SB3) are marked by the karstic surfaces are recognizable within the middle and upper portion of the Doushantuo successions in a shallow-water sedimentary setting. But these unconformities are missing in the deeper water setting,the corresponding sequence boundaries(SB2 and SB3)within the Doushantuo successions in the deeper water setting are represented by abrupt shift of lithofacies stacking patterns. The results demonstrate that the Doushantuo successions consist of two and half second-order sequences or super sequences with time span of ca. 35 Ma for SS1(635-600 Ma)and SS2(600-565 Ma),and ca. 14 Ma for SS3-TST(565-551 Ma). The cap dolomite distributed widely at the bottom of Doushantuo Formation and the maximum flooding surfaces in the sequences are important markers for stratigraphic correlation. Present sequence stratigraphic framework of the Doushantuo Formation can be used for stratigraphic correlation of the Doushantuo successions from the shallow-water facies to the deep-water facies of the Yangtze Plate. Meanwhile,our results indicate that the four members subdivision of the Doushantuo Formation based on its type succession in the Yangtze Gorges area is not practical for the whole Yangtze Plate and generally leads to incorrect stratigraphic correlations and confusions of the Doushantuo successions outside the Yangtze Gorges area. Therefore,it is suggested here that subdivision and correlation of the Doushantuo successions should be based on the second-order sequence stratigraphy together with carbon isotpe chemostratigraphy and biostratigraphy. The new stratigraphy framework of Doushantuo Formation will be a good basis for the palaeogeography evolution research and high-precision lithofacies palaeogeographic mapping.
Eboliang Ⅲ structure is one of the macrostructures in Qaidam Basin with trap area up to 470km2 in the Neogene Xiayoushashan Formation. In this paper,the authors studied the systematically the sedimentary environment evolution and provenance direction in Eboliang Ⅲ structure by using the new data of Wells Eshen 1,Eshen 2,and Eshen 7, which were drilled in succession at the east and west structural high points in recent three years. By synthetic analysis with petrology,palaeontology and geochemistry,some new points about the sedimentary environment evolution and provenance direction were obtained. The structure from the Shangganchaigou Age to Shizigou Age was under a deposition process with constantly deepening water, which was completely opposite to the previous viewpoint. Altun Mountains provenance provided main sedimentary clastics for Eboliang Ⅲ Structure. On this basis,the single well and multi well analysis has been developed to confirm the sedimentary evolution process,providing a theoretical basis for further exploration and deployment,and obtaining the corresponding exploration effect.
The Permian Lower Urho Formation overlapped layer by layer in the Block 8 of Karamay Oilfield, Junggar Basin. It suffered from intense denudation because of the late strong thrusting and uplifting. Therefore,resuming stratum palaeothickness not only can rebuild the burial and evolution history of the study area,but also is an important base of quantitative estimation of petroleum resources. However,traditional calculation method of denudation thickness is constrained by low degree of exploration. According to actual data and geological conditions of the study area,a suite of quantitative calculation method is probed out. Based on a new way of seismic trend thickness integrated with sedimentary sequences analysis,denudation boundary of the Permian Lower Urho Formation has been outlined. A composite method of the thickness ratio of nearby layers and the change ratio of reference layer thickness have been applied in calculating true denudation thickness of the Member 3 of Lower Urho Formation. By analyzing evolution characteristics of the Member 3 of Lower Urho Formation in the key sedimentary periods,a sedimentary model of fan delta has been established in the study area. The distribution range of delta front subfacies is also depicted at the same time,which can provide reliable geological evidence for the future petroleum exploration.
In order to meet the demand of arrangement,geosteering and track of the horizontal wells in Sulige Gasfield,Ordos Basin, the core survey,thin section appraisal,and analysis of vertical sequences and electrofacies were employed. Guided by depositional models of braided river,and based on the previous research results of the outcrops and modern quantitative reservoir sedimentology, the sedimentary microfacies and sandbody distribution in the horizontal well zone of the central Sulige area has been described finely. By analyzing the facies markers, sedimentary microfacies and depositional sequences of the lower Member 8of Xiashihezi Formation,as well as the detailed comparing of crowd well zone section, the statistics of reservoir sandbody thickness,and logging date of horizontal section in horizontal wells, the size parameters of reservoir sandbody are estimated. Three indexes(sand/stratum ratio,logging curve,reservoir sandbody) were studied out used to subdivide the sedimentary facies subfacies and microfaices. A map of sedimentary micro facies and sandbody distribution,which provides strong support for the horizontal well arrangement and geosteering.
Although high production oil has been achieved from continental shale of the Paleogene Hetaoyuan Formation by using large-scale hydraulic fracturing,less systematical research has been carried out on reservoir characteristics of the lacustrine shale in Biyang sag,Nanxiang Basin. In this paper,by use of thin sections and whole rock X-ray diffraction analyses,combining with the mineral composition feature of continental shale,the lacustrine shale oil in Biyang sag can be divided into massive mudstone,laminated argillaceous shale,laminated silty shale,laminated limestone shale and laminated dolomitic shale. With the photos taken by argon ion polishing high resolution scanning electron microscope(SEM),the reservoir characteristics in study area are concluded:(1)the main reservoir spaces include dissolution pore,intercrystal pore,intergranular pore,fracture,interlayer lamellation seam and macrostructure;(2)the pores are anisotropic;(3)mesopore is prior to micropore in study area,that is,the aperture size ranges from 0.841 to 213.34 nm with average in 4.76 nm, a main peak between 2 to 3nm and the second peak about 71 nm. It is indicated that rock type is the main factor controlling reservoir space development,and the corrosion pore,intercrystal pore,organic pore and interlayer lamellation seam are mainly developed in laminated limestone shale,which are beneficial to the shale oil accumulation.
