Characteristics and main controlling factors of tight oil reservoirs in Cretaceous Tengger Formation, A'nan Sag, Erlian Basin

Abstract

A set of "special lithology" tight reservoirs related to volcanic activity developed widely in the lower submember of the first member of Cretaceous Tengger Formation (K1bt1x) in the A'nan Sag, Erlian Basin, which is the main target for tight oil exploration. Based on section observation, physical property analysis, scanning electron microscopy (SEM), mercury intrusion porosimetry, CT scanning and nuclear magnetic resonance (NMR), the characteristics and main controlling factors of "sweet spots" of tight oil reservoirs with different lithologies in the K1bt1x were studied. Tuffaceous rocks, sandstones, dolomites and mudstones developed in the K1bt1x, and tuffaceous rocks and sandstones are the favorable lithologies for the development of tight oil reservoirs. The reservoir space consists of dissolved pores, intercrystalline micropores, residual intergranular pores, drainage pores and microfractures. The main reservoir space of tuff includes devitrified intercrystalline pores and dissolution pores, whereas that of sandstone includes residual intergranular pores and dissolution pores. The reservoir pore structures can be divided into 3 types and 4 subtypes. The thin-layered tuff and fine sandstone with solution pores have better pore structures, but the sedimentary tuff and dolomite have poor pore structures. The reservoir has a strong oil content heterogeneity controlled by reservoir lithology, porosity and permeability. The development of "sweet spots" in tuffaceous tight reservoirs is mainly controlled by the volcaniclastic deposition, devitrification and dissolution, while the development of "sweet spots" in sandstone reservoirs is mainly controlled by sedimentary microfacies, rock structure and dissolution.

Keywords

• tight oil reservoir
• geological characteristics
• main controlling factor
• a'nan sag
• erlian basin