Geological Factors Controlling Pore Throat Type Distribution in Lacustrine Tight Sandstone Reservoirs: the Chang 7 Tight Sandstone of the Ordos Basin, China

Abstract

Abstract

Tight oil is an important unconventional resource. The characteristics of tight sandstone are significantly different from those of conventional sandstone reservoirs. Compared with sandstones of conventional reservoirs, tight sandstone has lower porosity and permeability, more complex pore structures, and usually strong heterogeneity with respect to oil content.

The Yanchang Formation is composed of clastic sediments, and includes the Chang 7 member, which was deposited during a lake transgression maximum. The sands comprising the Chang 7 member were sandy debris flow sediments, which form a typical lacustrine tight oil reservoir, with a significant industrial output and high commercial value. Mercury injection and nuclear magnetic resonance (NMR) techniques were used, along with analysis of thin sections and casting sections, particle size analysis, X-ray diffraction (XRD)and scanning electron microscopy (SEM) to determine the effect of geological factors on the pore structures of Chang 7 tight sandstones.

Particle size analysis showed that the Chang 7 tight sandstones are composed of medium to poorly sorted fine-grained sandstone and siltstone. The median particle size ranged from 27.9 to 138.9 µm with the standard deviation varying from 1.25 to 2.38. Thin section and SEM observations indicated that the grains are mainly quartz and feldspar and that the interstitial material is mainly carbonates and clays. XRD analysis revealed that quartz and feldspar contents ranged from 50 to 75 vol%, clay minerals from 10 to 30 vol%, and carbonate minerals from 0 to 30 vol%. The pore types include residual intergranular pores, dissolved pores (including intragranular dissolved poresand mold pores), and inter-crystalline pores in clay minerals.

Both the mercury injection and NMR T₂ spectrum results showed that the median pore radius is positively correlated with quartz and feldspar content and with median particle size, whilst it is negatively correlated with the proportion of interstitial material (carbonate cements and clay). The grain size distribution and clay content of clastic rocks reflect the hydrodynamic conditions of sedimentation whilst carbonates indicates the extent of diagenesis. Factors such as particle size, mineral composition, and diagenesis are important for controlling the pore throat distribution in tight sandstone. These factors are ultimately controlled by the hydrodynamic conditions prevailing during sedimentation and by the degree of diagenesis.

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