Carbonate cementation patterns and diagenetic reservoir facies of the triassic Yanchang Formation deep-water sandstone in the huangling area of the Ordos Basin, northwest China

Abstract

The Chang 6 member sandstone is one of the main oil reservoirs of the Ordos basin, China, and consists of a lacustrine gravity flow deposited in Huangling area. Carbonate cement is volumetrically the most important diagenetic parameter controlling reservoir quality in the sandstones.Three generations of carbonate cements were recognized. The massive microcrystalline calcite, poikilotopic calcite and microcrystalline dolomite from the supersaturated alkaline pore water precipitate with basement cements at an early diagenesis stage. The ferroan calcite is mainly pore-filling and replacement cementation in the middle diagenesis stage, and ferroan dolomite is pore-filling cementation in the late diagenesis stage.The four types of reservoir facies and the distribution patterns were identified in the Chang 6 member sandstones. (1) Massive carbonate cemented facies with high contents of carbonate cements are distributed along the edges of the sandstone bodies, which are in the channels of the turbidite and sandy debris flow deposits. (2) Partial carbonate cemented facies occur along the margins of the massive carbonate cemented facies. (3) The porous facies with the lowest percentages of carbonate cements in sandstone are distributed in the middle of the sand-bodies of the sandy debris flow deposits. (4) High muddy debris facies that have high percentages of carbonate-cemented and high muddy debris sandstones occur essentially in the proximal overbank and lobe of the turbidite deposits.The reservoir quality of sandstones depended upon time of precipitation of carbonate cements on and diagenetic processes. Massive carbonate cements and the high muddy debris of sandstones undergo lots of carbonate cements and strong compaction in the early diagenesis stage, respectively. There was weak water rock reaction of dissolution in the later diagenesis process. Partial carbonate cemented facies and the porous facies of sandstones have sufficient water rock reaction and more dissolution pores.