Search

The Lishui Sag, in the East China Sea Shelf Basin, is rich in hydrocarbons, with the major hydrocarbon-bearing layers being the Paleocene Mingyuefeng clastic rocks. Analysis of the implicit geologic background information of these Paleocene clastic rocks using petrological and geochemical methods has significant practical importance. These Paleocene sandstones are mainly lithic arenite, lithic arkose and greywacke, composed of K-feldspar, plagioclase, authigenic clays, silica and carbonates. As continental deposits, Yueguifeng clastic rocks have high aluminosilicate and mafic detritus contents, while the Lingfeng and Mingyuefeng Formations are rich in silica due to an oscillating coastal marine depositional environment The major element contents of these Paleocene sandstones are low and have a concentrated distribution, indicating that the geochemical composition is non-epigenetic, transformed by sedimentary processes and diagenesis. The Yueguifeng detritus comprises recycled sediments, controlled by moderate weathering and erosion, while the Lingfeng and Mingyuefeng detritus is interpreted as primarily first-cycle materials due to low chemical weathering. In the Late Cretaceous to Early Paleocene, the Pacific Plate began subducting under the Eurasian Plate, causing an orogeny by plate collision and magma eruption due to the melting of subducted oceanic crust This resulted in the dual tectonic settings of 'active margin' and 'continental island arc' in the East China Sea Shelf Basin. During the Late Paleocene, the Pacific Plate margin migrated eastward along with development of the Philippine Ocean Plate, and the tectonic setting of the Lishui Sag gradually turned into a passive continental margin. Detrital sources included both orogenic continental blocks and continental island arcs, and the parent rocks are primarily felsic volcanic rocks and granites. [ABSTRACT FROM AUTHOR]

Gas-bearing deposits in the Lower Mingyuefeng Formation of Paleogene, Lishui Sag, East China Sea Shelf Basin consist of shoreface sandstones of the highstand systems tract (HST) and transgressive systems tract (TST), and deltaic sandstones of the lowstand systems tract (LST) and falling stage systems tract (FSST). Detailed petrographic observations suggest that the diagenetic features and related evolution of these deposits cannot be simply characterized and demonstrated in the depth domain. However, the occurrence of diagenetic minerals systematically depends on the studied interval within the HST, TST, LST, and FSST; therefore, diagenesis in this region can be better constrained when studied in the context of the depositional environments and sequence stratigraphic framework. The eogenetic processes in such settings include: (1) microcrystalline siderite precipitated as concretions in almost all environments and systems tracts, which inhibited further mechanical compaction; (2) grain dissolution and kaolinitization occurred in shoreface HST sandstones and deltaic LST and FSST sandstones; (3) glaucony was locally observed, which did not clearly reflect the controls of facies or sequence stratigraphy; and (4) cementation by pyrite aggregates occurred in the shoreface HST sandstones and deltaic LST sandstones. The mesogenetic diagenesis includes: (1) partial conversion of kaolinite into dickite in deltaic LST sandstones, and minor chlorite cementation in deltaic FSST sandstones; (2) transformation of kaolinite into illite and quartz cementation in deltaic LST and FSST sandstones; (3) frequent precipitation of ankerite and ferroan calcite in shoreface TST sandstones and early HST sandstones, forming baffles and barriers for fluid flow, with common calcite in shoreface HST sandstones as a late diagenetic cement; and (4) formation of dawsonite in the deltaic LST and FSST sandstones, which is interpreted to be a product of the invasion of a CO2-rich fluid, and acts as a good indicator of CO2-bearing reservoirs. This study has thus constructed a reliable conceptual model to describe the spatial and temporal distribution of diagenetic alterations. The results may provide an entirely new conceptual framework and methodology for successful gas exploration in the continental margins of offshore China, thus allowing us to predict and unravel the distribution and quality evolution of clastic reservoirs at a more detailed and reliable scale. [ABSTRACT FROM AUTHOR]