Determination of the Microstructure of a Lithologic Interface Using the Delayed Response Characteristics of Horizontal Well Gamma Ray Logging Curves: A Case Study of the Daqingzijing Oilfield, Songliao Basin, Northeast China

Abstract

Most vertical wells pass through a mudstone interface at large angles (70–90 $$^\circ $$ ), whereas the contact angle between a horizontal well and a lithologic interface is generally relatively small (20–50 $$^\circ $$ ). Therefore, in the vicinity of this contact point, certain interfaces can be detected through a gamma curve (less than 45 cm), and these will cause a delayed response in the curve. Using the information obtained from the delayed response, the fluctuation characteristics of the lithologic contact interface can be characterized. This study examines gamma ray curves for 122 horizontal wells in the Songliao basin. A detailed analysis of the characteristics of the delayed response when the well passes through the interface between sand and mudstone is conducted, and the lengths of the delay intervals are determined for all wells. An equation for calculating the stratigraphic dip angle of the horizontal well is derived based on the drilling inclination angle in the logging process, and the true dip angle of the contact point is obtained. From these results, a plane distribution map of the dip angles in the study area is drawn and compared with the log interpretation results. Only slight differences are found, which demonstrates that the proposed method has a high reliability. Based on these results, a microstructural map of the fine sand top surface of the HDM reservoir in the Songliao basin is drawn (contour distance: 2 m), which can guide the deep development of the oilfield.