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Based on the latest drilling cores, thin sections, and 3D seismic data, types, features, and evolution processes of Cambrian–Ordovician carbonate platforms in the Gucheng and Xiaotang areas are studied, and favorable exploration zones in this area are also discussed. There are two types of carbonate platforms developed in the Cambrian–Ordovician in the Gucheng–Xiaotang area, namely, carbonate ramp and rimmed platforms, and the evolution process of the platform in the Gucheng area is different from that in Xiaotang area. In the Early Cambrian, the study area was a homoclinal carbonate ramp. During the Middle to Late Cambrian, it evolved into a rimmed platform, with 5 phases of mound shoals developed. In the southern area, mound shoals were in progradational arrangement and the third and fourth stages of mound shoals suffering exposure and further developed abundant karst vugs. In the northern area, the mound shoals appeared in a superposition of aggradation–weak progradation, the third, fourth, and fifth stages of mound shoals suffered exposure and dissolution, and the platform slope developed gravity flow deposits. In the Early to Middle Ordovician, the southern area gradually evolved into a distally steepened carbonate ramp, where retrogradational dolomitic shoal developed; while the northern part experienced an evolution process from a weakly rimmed platform to a distally steepened carbonate ramp, and developed two or three stages of retrogradational mound shoals. The high-frequency oscillation of sea level and local exposure and dissolution were beneficial to the formation of mound or shoal reservoirs in platform margin and ramp, and the configuration of these reservoirs with low energy slope–basin facies source rocks could form good oil-gas enrichment zones. The dolomitic shoal in the Ordovician platform ramp is the practical exploration field for increasing reserve and production in the Gucheng area. The mound shoal at the Cambrian rimmed platform margin is the key exploration object in the Xiaotang area. In addition, the Cambrian slope gravity flow deposits can be taken as the favorable exploration fields in the study area.

Based on the latest drilling core, thin section, 3D seismic, well logging data as well as exploration results, the sequence stratigraphy and sedimentary microfacies of the Middle—Lower Ordovician carbonates in Gucheng area, and their controlling effects on the development of reservoir were examined by the theories and methods of fine carbonate sedimentology. The results show that the Middle—Lower Ordovician in Gucheng area is a set of typical carbonate ramp deposits, which can be divided into 10 microfacies in 4 subfacies as follows: back ramp, inner shallow ramp, outer shallow ramp, and deep ramp. The back ramp subfacies consists of muddy-dolomitic flat and dolomitic lagoon microfacies; it is dominated by lamellar micrite dolomite tight in lithology. The inner shallow ramp subfacies includes dolomitic shoal and dolomitic flat in shoal top and dolomitic flat between shoals microfacies; it is mainly composed of crystal dolomite with metasomatic residual structure, as well as abundant karst vugs and intercrystalline pores. The crystal dolomite has an average porosity of 4.36%. The outer ramp subfacies includes medium—high and low energy grain shoal and inter-shoal microfacies; it is dominated by sandy limestone, oolitic limestone, and micrite limestone with few pores. The deep ramp subfacies is dominated by low-energy argillaceous deposits, with local presences of storm shoal microfacies. The Lower—Middle Ordovician has six third-order sequences from bottom to top, among which sequence (SQ3) represents the third member of Yingying Formation. The main reservoir has three fourth-order high-frequency sequences. Apparently, the favorable reservoir in the study area is jointly controlled by sedimentary microfacies and high frequency sequence in the carbonate ramp. The former controls the primary pore structure, and the latter controls the intensities of karstification exposure and dolomitization in the penecontemporaneous period. The dolomitic shoals and top dolomitic flats of different stages, superimposed and connected into favorable reservoirs of considerable scale like “platform margin”, are favorable exploration facies in the carbonate ramp.

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Copyright of Natural Gas Geoscience is the property of Natural Gas Geoscience and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)