Integrated Factors Controlling the Characteristics of Tight Oil Reservoir of the Lucaogou Formation in the Jimsar Sag of Junggar Basin, Northwest China

Abstract

Abstract

The research methods of reservoir characterization have been developed very fast in recent years, from macro-scale to nano-scale, from 2-D scale to 3-D scale, and the research precision has also been largely improved. In this study, the Rock Thin Section, FIB-SEM, Energy Dispersive Spectrometer and X-ray Computed Tomography were employed to characterize the reservoir properties in Lucaogou Formation, aiming at clarifying key factors that affect those properties considerably.

Lucaogou Formation is composed of various types of rocks with a sequence of mudstone, dolomitic sandstone, detrital dolomite, dolomite and tuffite precipitated in saline lacustrine basin, having diversified sediment sources from the near surroundings of the sag. The internal open porosity of the reservoir ranges 5∼18%, averaged in 10%, the permeability ranges 0.001∼10 mD, averaged in 0.06 mD. The poor properties of the reservoir are mainly controlled by diagenesis and hydrocarbon generation on the basis of sedimentation. Low compositional and structural maturity is evidenced to be in favor of the diagenetic processes, elucidating sedimentation is a key factor to the evolution of reservoir properties. Images shows that 90% of the pores are mainly primary secondary solution pores and remnant intergranular pores, while less than 10% of which are fractures. The majority of primary pores are destroyed by mechanical compaction where fine-grained mineral and organic components undergo rearrangement. Debris, dolomite and feldspar dissolution and creation of secondary pores increase porosity, whereas precipitation of fine-grained clay and silica cements reduce primary porosity. Meanwhile, intragranular pores take place in the authigenic minerals which are proved to be interconnected by the existence of bitumen in these pores. Images acquired from X-ray computed tomography manifest microcracks are significant pathways for hydrocarbon migration which are the connecting path between pores. Those microcracks are along minerals rather than through the particles which might be productions of hydrocarbon generation instead of tectonic effects. It is proved to be easier for the microcracks forming along the minerals than through the particles since cementation of the reservoirs is weak.

AAPG Datapages/Search and Discovery Article #90260 © 2016 AAPG/SEG International Conference & Exhibition, Cancun, Mexico, September 6-9, 2016