Diagenetic Controls on Reservoir Quality in Lower Cretaceous Bashijiqike Sandstones From Ultradeep Burial (Over 6000m), Kuqa Foreland Basin, China

Abstract

Reservoir quality is a critical risk factor in deep to ultradeep reservoirs at depths >4.5 km. Analysis of lower Cretaceous Bashijiqike sandstones in the Kuqa foreland basin provides insight into the evolution of reservoir quality during deep to ultradeep burial diagenesis. Reduction of porosity and permeability with burial in Bashijiqike sandstones was evaluated using subsurface samples from 6000 to 8000 m(Keshen gas field), at temperatures of 150~200 Celsius. Diagenesis and petrophysical properties were interpreted from petrographic data and core analyses. Bashijiqike sandstones are mostly lithic arkoses and feldspathic litharenites having an average composition of Q41F34R25. Provenance did not change significantly during Bashijiqike deposition in this area, nor does average sandstone composition vary among lower, middle, and upper Bashijiqike sandstones. Pore types in Bashijiqike sandstones change from a mix of primary and secondary pores and micropores at deep depths to predominantly secondary pores and micropores in deeper sandstones. Bashijiqike sandstones show steady porosity reduction from an average of 12% at 142 Celsius to 6% at 197 Celsius, with little additional loss at higher temperatures. By 197 Celsius, most primary pores have been lost by mechanical compaction or occluded by quartz cementation. Volume of secondary pores remains nearly constant despite dissolution of 3.5% average potassium feldspar during deep burial. Volume of this late dissolution of potassium feldspar is offset by precipitation of ankerite, albite, illite, and minor quartz in secondary pores. The proportion of microporosity, which occurs in authigenic clays, altered grains, and matrix, increases in the deepest sandstones. Change in proportion of pore types and reduction in total porosity with increasing temperature changes porosity–permeability transforms. Because most deep to ultradeep. Bashijiqike sandstones are dominated by secondary pores and micropores, porosity–permeability transforms have lower slopes than those for shallower sandstones(KL2 gas field).

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015