Diagenetic sequence and genetic mechanism of Silurian tight sandstone reservoirs in the eastern Tarim Basin, Northwest China

Abstract

The Silurian stratigraphic sequence has recently become one of the most important exploration targets in the Tarim Basin, with a considerable amount of profitable hydrocarbon pools discovered in the central Tarim Basin. Previous exploration activities indicate that the Silurian stratigraphic sequence in the eastern Tarim Basin has great hydrocarbon exploration potential. The Silurian reservoirs comprise a set of tight marine sandstones, whose diagenetic sequence and genetic mechanism are still poorly understood. The complex relationship of hydrocarbon generation, the timing of the peak expulsion of the source rocks and the evolution of the reservoirs remains unclear. An integrated description and analysis have been carried out on core samples from eleven wells selected from the eastern Tarim Basin. A range of petrographic and geochemical analyses were conducted. By using an integrated approach with thin-section petrography, scanning electron microscopy (SEM), cathodoluminescence (CL), carbon and oxygen isotope geochemistry, formation water analysis, X-ray diffractometry (XRD), electron probe microanalysis and fluid inclusion microthermometry, the genesis and occurrence of individual diagenetic events were documented to reconstruct the diagenetic sequence and diagenetic model for the Silurian sandstone. Additionally, the tight nature of the Silurian reservoirs can mainly be attributed to the compaction processes and cementation. In particular, the destructiveness of the compactional processes to the original porosity is far greater than that from the cementation. Furthermore, fluid inclusion analyses also indicate that the Silurian sandstone has experienced three phases of hydrocarbon charge. The first two phases occurred during the eodiagenesis stage (from the Late Silurian to the Early Devonian and from the Late Carboniferous to the end of the Late Permian), when the Silurian sandstone was not tight and had a porosity of greater than 20%. The third phase occurred during the stage B of mesodiagenesis (since the Late Cretaceous), when the Silurian sandstone was fully tight.

Keywords

eastern Tarim Basin / Silurian sandstone / diagenetic sequence / hydrocarbon charge / genetic mechanism

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