Occurrence and origin of pore-lining chlorite and its effectiveness on preserving porosity in sandstone of the middle Yanchang Formation in the southwest Ordos Basin

Abstract

The occurrence and origin of pore-lining chlorites in sandstone were studied using mineralogical and crystal chemical analysis and their effectiveness on preserving porosity was discussed. During the sedimentary period of the middle Yanchang Formation, the two provenance and sedimentary systems of the southwest and northeast developed in the southwest Ordos Basin. In the Northeast of the study area, the content of biotite flakes is approximately 10vol%. Pore-lining chlorites prefer to grow on the surfaces of fine-grained detrital quartz and feldspar but not on ductile grains such as biotite. Pore-lining chlorites that developed during the early stage of eodiagenesis (<90°C) probably locally increased the compressive strength of the rock, in places with residual intergranular pores. The chlorites within different sand bodies show slight variations in their chemical composition. After initial mechanical compaction, the formation of authigenic chlorites (brunsvigite and diabantite) was probably related to the hydrolysis and alteration of biotite fragments and the migration of diagenetic fluid. In the Northeast, only 2vol% of the chlorite rims significantly inhibited quartz overgrowth, preserving the primary porosity. However, they probably blocked and delayed the dissolution of acids to feldspar. The porosity probably decreased when the authigenic chlorite content exceeded 9.38vol%. In the Southwest, the authigenic chlorite is rare and quartz-overgrown cement is widespread, causing poor reservoir quality. Authigenic chlorites increase with the content of biotite flakes in the northeast and southwestern parts of the study area, but excess ductile biotite flakes and authigenic chlorites have negative effects on reservoir quality.