| Author's Name: LIU Puyu1,2, GONG Chenglin1,2, PENG Yang1,2, HUANG Xiaobo3,
WANG Qiming3, LI Dongwei1,2, WANG Haiyi1,2 |
| High-quality lacustrine source rock and extensive sublacustrine fans are developed in deep-lacustrine
depositional environment in lower section of the second member of Oligocene Dongying Formation (E3d2l) in southern
Bozhong Sag, providing an exploration realm of lithological reservoirs. However, the generally poor porosity and
permeability of the deep reservoirs severely hampered the progress for oil and gas exploration and development. Thus,
there is an urgent need to elucidate the key factors and the pore evolutionary processes controlling the sandstone reservoir
quality of the sublacustrine fans. Based on comprehensive analysis of drilling and logging data, rock grain-size, thin
sections, scanning electron microscopy and fluid inclusions, this study has successfully identified the key factors
controlling reservoir quality, and established quantitative models of pore evolution. The results show that: The
sublacustrine fan reservoir of the E3d2l is characterized by predominantly medium-fine grained lithic feldspathic quartz
sandstone with mainly dissolution pores. The favorable sedimentary units such as lobe axis and distributary channels in
sublacustrine fan are the basis for the formation of high-quality reservoirs. Persistent compaction and early / late
cementation are the main causes of continuous porosity loss. Organic acid dissolution and abnormally high pressure are
the key factors for improving reservoir quality. The sublacustrine fan sandstone undergoes four stages during the burial
process: rapid compaction in the early diagenesis A phase, rapid compaction and early cementation in the early diagenesis
B phase, slow compaction and organic acid dissolution in the middle diagenesis A1 phase, and slow compaction and late
cementation in the middle diagenesis A2 phase. During these phases the compaction and early cementation reduce porosity
by 13.64% and 8.24% respectively, and organic acid dissolution increases porosity by 9.55%. In particular, the late
cementation after the oil and gas accumulation results in a severe porosity decrease of 7.12%. This study will provide a
theoretical basis for the oil and gas exploration and development of sublacustrine fan reservoirs in the deep-lacustrine
basin of the Bohai Sea. |
