Reservoir characteristics in the LW3-1 structure in the deepwater area of the Baiyun sag, South China Sea

Abstract

Geochemistry including carbon isotopes, biomarkers, and maturity indexes indicates that the oil and gas in the Liwan 3-1 (LW3-1) structure are derived from source beds in the Enping, Wenchang, and Zhuhai formations (fms.). The major contribution is from the Enping fm. Two major sandstone reservoirs in the LW3-1 structure are present in the early Miocene deepwater turbidite fan of the Zhujiang fm. and the late Oligocene delta front sediments of the Zhuhai fm., where most traps are structural traps and drape anticlines. Faults act as the main migration pathways while permeable sandstone layers are favorable lateral passage systems. Structures formed by diapirism are conducive to oil and gas migration. A combination of faults, the location of the diapir belt, and the structural ridge control the oil and gas migration and accumulation. Sand units 1–3 in the LW3-1 structure share a common temperature-pressure system and gas-oil contact; they are considered to be the same oil- and gas-bearing strata set. Oil and gas injection intensity is higher in sands 1 and 2 than in sands 3 and 4, in which sand 4 is separated from the upper three reservoirs and belongs to a different oil- and gas-bearing strata set. Oil and gas injected in sands 1–3 migrate vertically along faults, and then migrate laterally to accumulate in highly permeable sands. In sand 4, oil and gas migrate along faults; they accumulate in the trap and form a reservoir with better perseveration of oil and gas compared with the others.