Distribution and Origin of Carbonate Cements in Paleogene Nearshore Subaqueous Lacustrine Fans of Dongying Depression of Bohai Bay Basin in China

Zhang, Liqiang; Yang, Wan; Luo, Xiaorong; Gao, Yongmin; Liu, Shuihui; Luo, Hongmei

Carbonate cementation affects quality of unconventional sandstone reservoirs in deep lacustrine basins. Sandstones of nearshore subaqueous fans in the upper 4th Member of Paleogene Shahejie Fm. in Yongan, northern steep margin of Dongying Depression were studied to understand the effects, using 1000 m of cores, wireline logs, and 150 thin-sections of 18 wells. Sandstones are gravelly, fine to coarse, poorly-moderately sorted, and compositionally and texturally immature as lithic and feldspathic wackes. Plagioclase is abundant. Volcanic, metamorphic, and carbonate lithics are common in different parts of the basin. Calcite, ferroan calcite, dolomite, and ankerite are main cement minerals, averaging 7%. They are subhedral-euhedral, 20-50m in size, blocky, isopachous, and poikilotopic. Diagenetic sequence is from compaction, calcite-dolomite cementation, feldspar dissolution, quartz overgrowth, ferroan calcite cementation and quartz dissolution, calcite and feldspar dissolution, to ankerite cementation. Distribution and thickness of cement zones were interpreted from cores and wireline log signatures of low porosity and permeability. Carbonate-cemented sandstones occur mainly in: 1) thin, fine sandstones intercalated with shale in distal fan facies; 2) basal parts of coarse channel sandstones in mid-fan; and 3) carbonate clast-rich conglomerates in upper fan. Cement zones are <1 m thick and up to 10 km long in distal fans, mainly in sandstones underneath maximum-flooding deposits. Those in mid-fans compartmentalize non-cemented sandstones. Amount of carbonate clasts in upper-fan conglomerates varies laterally and causes heterogeneous distribution of cement zones. Carbon and oxygen isotopic compositions of cement-rich whole-rock samples vary greatly (δ13CPDB=-7.2 - +8.5,δ18OPDB=-5.9 - -16.2‰) and, combining with petrographic and stratigraphic data, indicate variable sources of calcium carbonate from dissolved limestone and organic shale, and influence of acidic fluids. Negative correlation between amount of plagioclase and carbonate cements suggests that plagioclase dissolution by acidic water from organic maturation buffered the water, leading to carbonate precipitation at the edge of sand bodies. Finally, negative correlation between degree of cementation and porosity-permeability is established. Our results indicate carbonate cementation as the main cause of strong reservoir heterogeneity and lithologic traps in distal fan deposits.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013