An Analysis of the Changing Rules of Water-flooded Reservoir Controlled by Rock Facies and Sedimentary Microfacies: A Case from Donghe Sandstone in Donghe1 Well Area , Tarim Basin, Northwest China

Coastal reservoir is relatively homogeneous. However, reservoir presents obvious difference after perennial water flooding. Donghe sandstone experienced weak diagenesis effect. The changing rules of water-flooded reservoir is... [ view full abstract ]

Coastal reservoir is relatively homogeneous. However, reservoir presents obvious difference after perennial water flooding. Donghe sandstone experienced weak diagenesis effect. The changing rules of water-flooded reservoir is mainly controlled by sedimentary factors. Delta front deposits in Donghe1 well area developed in the early carboniferous, and then was destroyed by strong transgression. The coastal deposits have retained the sedimentary characteristics of delta fronts due not merely to the certain scale of early delta front sediments, but to the fast sedimentary rate of Donghe sandstone, which are coastal sediments on the background of delta. There are seven types of rock facies that have been further classified in the study area: branch channel transformation shoreface dam(A); mouth bar transformation shoreface dam(B1); normal shoreface dam(B2); delta transformation shoreface beach(B3); normal shoreface beach(B4); sand sheet transformation offshore sand(C1); normal offshore sand(C2). The main pay zone in study area is located in shoreface and offshore. Shoreface dam is well developed in shoreface, including A, B1 and B2 rock facies. (A) keeps positive rhythm in vertical direction and appears stripped in the plane. (B1) mainly develops at the end of (A). (B2) is row-shaped, paralleling to the direction of paleoshoreline. Shoreface beach is mainly distributed at the thinning edges of shoreface dam, including B3 and B4 rock facies. (B3) generally is adjacent to (A) and (B1). (B4) mainly develops near (B2). Hydrodynamic force is relatively weak in offshore which includes C1 and C2 rock facies. (C1) yields cross bedding, and the sand thickness reduces along the direction of the sea. (C2) is composed by the combination of interbeded mud and sandstone. Reservoir composition, structure and physical property of different rock facies have changed dramatically after water flooding. The content of clay mineral, such as illite, kaolinite and illite/smectite decrease after (A) has been water flooded. Furthermore, contact relation of particles converts from line-contact to point-contact, cementation type from pore-cementation to contact-cementation. Both the reservoir porosity and permeability increase after water flooding. The content of clay mineral bear unconscious changes in (B1~B4) after water flooding. In low water stage, contact-cementation predominates, but in high water stage, pore-filling cementation begins to increase. As the water flooding goes on, porosity first increases and then decreases, but permeability shows a decreasing trend. The reservoir sensitivity of (C1~C2) is strong. After water flooding, the existing form of clay mineral converts to bridge type and pore-filling type and reservoir properties deteriorate.