Sequence stratigraphic characteristics and subtle-trap exploration of the sagged lacustrine basins

摘要

The sagged lacustrine basins are widely distributed in China. Three types of sagged lacustrine basins are classified on the basis of geologic settings of basin formation. (1) The sagged basins are developed on former rifted basins, such as the Songliao basin; (2) the sagged basins like the Ordos basin evolved on the basis of paleo-cratonic basins; and (3) the sagged basins, such as the Junggar basin, developed from the Late Paleozoic marine-basin closing. The Mesozoic sagged lacustrine basins are one of the most important oil-bearing basin types in China. Terrigenous clastic sediments of the sagged lacustrine basins are supplied by fluvial and alluvial fan systems around the basins. Delta systems concentrate on one depocenter or several depocenters. Lacustrine basins are alternately filled with regressive and transgressive depositional intervals. The lacustrine mudstones deposited during maximum flooding periods are important source rocks and regional seal beds. The sandstone bodies interbedded with the mudstones deposited two maximum flooding periods are most important targets for searching for the subtle traps. There are many higher-order cycles of lacustrine level changes between two major maximum flooding periods. Such frequent changes of lacustrine level and sediment supply result in shoreline migrating and delta-lobe shifting. Thus, delta front belts become favorable areas of searching for lithologic oil reservoirs. The subtle oil reservoirs in the sagged lacustrine basins are generally characterized by wide distribution, low permeability, and low richness, which may form high-accumulative and high-productive blocks due to matching with favorable structural settings. The Qingshankou, Yaojia, and Nenjiang (QYN) oil-bearing interval in the Songliao basin can be illustrated as one typical example.