PORE‐LINING CHLORITE CEMENTS IN LACUSTRINE‐DELTAIC SANDSTONES FROM THE UPPER TRIASSIC YANCHANG FORMATION, ORDOS BASIN, CHINA

Abstract

Non‐marine sandstones in the upper Triassic Yanchang Formation constitute an important reservoir rock in the Zhenjing area of SW Ordos Basin, Central China. A study of the diagenetic history of these sandstones, including the formation of pore‐lining chlorite cements, was undertaken to investigate controls on reservoir quality. Data came from the petrographic analysis of cores from five wells. The sandstones were deposited in a lacustrine‐deltaic setting and consist of fine‐to medium‐grained feldspathic litharenites and lithic arkoses, and in general have low porosity and permeability. The principal diagenetic processes observed were compaction, cementation, dissolution and replacement. Mechanical compaction and calcite cementation caused a reduction in reservoir quality. The precipitation of pore‐lining chlorite inhibited quartz cementation and preserved porosities of up to 16%. The chlorite occurs in a double layer which lines pore spaces, particularly in distributary channel sandstones. Both chlorite layers grew by precipitation from pore waters containing the products of dissolution of volcanic rock fragments, biotites and feldspars. An increase of temperature during diagenesis was probably the cause of the two generations of chlorite cement, and also of the observed variations in elemental concentrations between the earlier‐ and later‐formed chlorite. The chlorite was precipitated after early compaction but before other diagenetic phases.