Fracture Characterization at Multiple Scales Integrating Geologic, Borehole and Seismic Data in Tight Oil Sandstones

Abstract

Abstract

Tight oil sandstones are distinguished by the presence of multi-scale fractures, ranging from several micrometers to hundreds of meters, while, fractures at different scales could be described by different approaches and have different contributions to tight oil. Based on the mechanical stratigraphy boundaries controlling fracture development, fractures within reservoirs are divided into micro-scale, small-scale, meso-scale and macro-scale ones. Taking the tight oil sandstones of the Upper Triassic Yanchang Formation in southwest Ordos Basin, China as an example, micro-scale fractures are characterized by using casting thin sections, scanning electron microscope and CT scanning. Small-scale fractures are described by cores, borehole resistivity or acoustic image logs and pre-stack seismic amplitude-azimuth anisotropy. Combined with pre-stack seismic attenuation-azimuth anisotropy, cores, multi-pole array acoustic logs and azimuthal acoustic reflection logs are used for meso-scale fracture characterization. Macro-scale fractures are characterized by post-stack seismic attributes, such as coherence and variance. The results show micro-scale fractures, with lengths of centimeters or less than centimeters, are present in a single sandbody. Small-scale fractures with lengths of several to tens of meters and heights of several decimeters to meters, are developed within a single sandbody and generally normal to bedding planes. Meso-scale fractures, with heights of several to ten meters and lengths of tens of meters, are developed in a parasequence, cut bedding planes and are bounded by thin interlayers. Controlled by barrier layers, macro-scale fractures, with heights of tens of meters and lengths of hundreds of meters, are developed in parasequence sets and cut interlayers. Integrating production data and multi-scale fracture physical properties, multi-scale fracture contributions to tight oil are evaluated. Micro-scale fractures, as important storage space, connect pores and enhance the connectivity of tight oil sandstones. Small-scale and meso-scale fractures are effective storage space and seepage channels for tight oil sandstones. Macro-scale fractures, as the flow pathways, increase the integral seepage capability. This study provides an example for the characterization of multi-scale fractures and unravels their different contributions to tight oil sandstones which provide a geological basis for tight oil exploration and efficient development.

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