--> An Integrated Sedimentological and Petrophysical Evaluation of the Cretaceous Oil Sands in the Fengcheng Area, Jungar Basin, NW China

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An Integrated Sedimentological and Petrophysical Evaluation of the Cretaceous Oil Sands in the Fengcheng Area, Jungar Basin, NW China

Abstract

The Cretaceous Fengcheng oil sand in the northwestern margin of the Jungar Basin, NW China is a major heavy oil producing reservoir in China. It comprises three reservoir units, namely, No. 1, No. 2, No. 3 oil sands. The reservoir can be divided into three sections stratigraphically: Sections K1q3, K1q2, and K1q1, developed on paleo monoclinal structural highs striking NE-SW. The base of the Cretaceous (K1q3) is composed of sandy conglomerate and pebbly sandstone. The K1q2 oil sand in the middle comprises sandstone. The top K1q1 oil sand consists primarily of fine sandstone. The No. 1 oil sand is of braided river delta front deposition, whereas the No. 2 and No. 3 oil sands are of braided river delta plain deposition. The oil sands contain primary intergranular pore, residual intergranular pore and minor intragranular dissolved pores. The average porosity and permeability of the K1q2 oil-bearing sandstone is 34.2%, and 1090 mD, respectively. The oil density is 0.9789 g/cm3, while the surface viscosity at 50 degree C is 266,000 mPa.s. The No. 1 oil sand is typical of a lithological-structural reservoir, while the No. 2 and No. 3 oil sands are of structural-lithological reservoirs. The burial depths of the oil sands have been always shallow, < 200 m. The reservoir temperature is 18–20 degree C, and the pressure is 1–2.3 MPa. The major oil sand pay zones are of channel bars and underwater inter-distributary channels. The oil sand reservoir has the following characteristics: (1) Reservoirs in proximity to faults and unconformity surfaces have higher oil saturation; (2) Higher oil saturation corresponds to reservoir units with better porosity and permeability; (3) The Deeply buried oil sands have better oil saturation.