The effects of paleogeomorphology on the development of fractures in reservoirs of weathering metamorphic zone in an exposed Archean burial hill, Block B, Bohai Bay Basin

Burial hill is one of the important targets for oil and gas exploration and development in the Bohai Bay Basin where Block B in an exposed Archean burial hill is recently found to be hosting the largest gas condensate field in eastern China. The paleo-geomorphology is suggested to have played an important role in the formation and development of reservoirs in the field that caused differences in duration and intensity of weathering upon the burial hill of metamorphic rocks and thus shaped the development of reservoirs and even the formation of fractures in the weathering zone of the burial hill. Based on core description and image logging interpretation of fractures as well as the restoration and division of the ancient landform of the burial hill, the control of paleogeomorphology on the formation of fractures in the reservoirs of burial hill is discussed in detail. The following results are obtained: ① The weathering and leaching effect is relatively stronger at the higher parts of the paleogeomorphology, resulting in denser fractures. The paleogeomorphology directly affects the development of fractures in the weathering zone, weakens the rock properties of the reservoir in the inner zone, and indirectly controls the development of fractures in the inner zone to a certain extent. ② The depth ranges of effects of paleogeomorphology on weathering zone of buried hill are different in different areas, which controlled the formation of fractures in different depth of the burial hill. The paleogeomorphology in the east part of Block B is very different from that in the west part, with the thickness of the weathering fracture zone in the west part (277 m) significantly larger than that in the eastern part (193 m). The depth of the weathering fracture zone affected by the paleogeomorphology in the west part is also greater than that in the east part. ③ There is a clear correlation between paleogeomorphology and the fracture density of each fracture group/system (stage), i. e., the fracture development in the weathering zone is not only controlled by paleogeomorphology, but also related to the tectonic activities of each period, especially the early tectonic activities (Indosinian period) which controls jointly with paleogeomorphology the formation of early fractures in the weathering zone. ④ Block B contains 3 large high-pitched ridge remnant mound areas with well-developed fractures. Fractures, especially the NW-SE-trending fractures of Indosinian period in the weathering zone are controlled by geomorphologic amplitude. More complete water-rocks reaction occurs in the gentle flanks of the remnant mounds, where the weathering and leaching effect is strong enough to reduce the mechanical properties of the rocks and facilitate the formation of fractures under various stresses and large-scale dissolution, all providing favorable conditions for the formation of high-quality weathering zone reservoirs.