Utilizing microresistivity image logs to recognize conglomeratic channel architectural elements of Baikouquan Formation in slope of Mahu Sag

Abstract

Geological outcrop is usually the first-hand evidence to cognize the probable geometry, lateral extent, and other properties of conglomeratic channels. If there is no exposure of formation, however, it is hard to recognize elaborate architectural elements just based on poor coverage cores and low-resolution conventional logs vertically. In order to understand vertical distributions of conglomeratic channel architectural elements in the Lower Triassic Baikouquan Formation in southern slope of Mahu Sag, ultrahigh-resolution microresistivity image logs are used to recognize elements under the concept of conglomeratic channel architectures in this article. The microresistivity images were calibrated with cores in different grain-sizes and sedimentary structures. According to microresistivity image fabric, subaerial channel-fills consisted of granules to cobbles; subaqueous channel-fills were composed of well-sorted sandstones to granules. Subaerial channel-margins comprised sandstones to pebbles; subaqueous channel-margins were granules and sandstones. Bypass drapes, deposited from the tail of bypassing flows, were commonly eroded by overlying channel-fills. Convergent drapes, accumulated on top surfaces of channel-margins, preserved well without reworking. Overbank drapes, formed outside the channel, were thick-bedded. Combined with dipmeter pattern of microresistivity image log, six channel story sets were recognized depending on abrupt changes in dip azimuths. In each channel story set, per unit thickness of mud drapes ranged from 0.1 to 0.6 m. Mud drapes in channel story set 1 could be correlated horizontally. In channel story sets 3, 4, and 5, thickness of mud drapes ranged from 0.05 to 2.59 m and averaged at 0.59 m, and channel conglomerates and mud drapes alternated frequently. These understandings would provide important insights on the production potential of petroleum.

Keywords

• microresistivity image log
• conglomeratic channel
• architectural element
• mud drape
• baikouquan formation
• mahu sag