219 / 2019-06-25 16:39:08

Diagenetic facies and reservoir quality prediction via well logs—a case of Dongying Formation Member 1 of Nanpu Sag

diagenetic facies; sandstones; logging identification; Dongying Formation; RQI;

The paleogene Dongying formation in Nanpu sag of Bohai Bay Basin is a fan delta-braided river delta front subfacies deposition, with relatively good reservoir physical properties and great oil and gas exploration potential. On the basis of core observation, the composition, texture, pore systems, reservoir property, and diagenetic modifications of the first member of Dongying formation in Nanpu sag are systematically analyzed and studied by means of thin section petrography, X-ray diffraction (XRD), scanning electron microscopy (SEM), routine core analysis (RCA), MICP (mercury injection capillary pressure) and NMR (nuclear magnetic resonance) tests. The results show that the reservoirs are mainly feldspar lithic sandstone, their structure and composition are immature, the pore systems consist of primary intergranular pores, intergranular and intragranular dissolution pores, moldic pores and micro-pores associated with authigenic clays. The reservoirs are now in the middle diagenetic evolution stage A, and the main diagenetic sequence is compaction—kaolinite and smectite cementation—early carbonate cementation—quartz overgrows—hydrocarbon filling—feldspar dissolution—clay mineral filling—carbonate cementation. The sandstones experienced low to moderate mechanical compaction, but high degree of framework dissolution, and the diagenetic minerals are dominantly of carbonates (calcite, dolomite, and Fe-dolomite) and clay minerals (illite, mixed layer illite/smectite and kaolinite). Based on the above features, diagentic facies can be divided into four types: tightly compacted facies, carbonate cemented facies, clay mineral filling facies, and dissolution facies. Through routine core analyses, the range of porosity permeability and oil saturation of these diagenetic facies was determinded. The diagenetic facies were then translated to conventional well logs by core-log calibration, and the well log expressions (in terms of natural gamma ray, neutron porosity, bulk density, sonic transit time, and resistivity logs) of various diagenetic facies were summarized. The mineralogical content obtained from the Elemental Capture Spectroscopy (ECS) log was then used for the diagenetic facies prediction, and the diagenetic facies predicted with the comprehensive conventional log analyses were combined with the ECS log profile, and were calibrated with the XRD, SEM analysis and logging interpretation results. For wells without ECS log, the diagenetic facies can be predicted by the calculation of RQI (reservoir quality index) via the GR, AC and DEN log curves. Reservoir quality or oil bearing properties to some extent can be evaluated and predicted by translating the diagenetic facies to well logs.