Litho-stratigraphic reservoirs are difficult to evaluate due to the complexity of formations and traps. To cope with such difficulties, ten essential elements that influence trap development have been identified and technical procedures for trap evaluation have been established. In addition, based on the application of "Structure-Sequence Assemblage of Reservoir Formation Model", a new method—"Four Map Overlay" for litho-stratigraphic reservoir evaluation—has been developed.

Such technological innovations lead to the establishment of new standards for classifying and evaluating litho-stratigraphic oil and gas reservoir accumulation zones. The result is that several 100 million ton grade oil and gas fields were discovered, including the deep zone of Songliao Basin, Xifeng field in Ordos Basin, Chuanzhong field in Sichuan Basin, Tazhong field in Tarim Basin, and the northwestern edge of Junggar Basin. The newly discovered oil and natural gas reserves from these litho-stratigraphic reservoirs account for 60% and 80% of our total reserve increase respectively.

"Four-Map Overlay" evaluation technique for litho-stratigraphic reservoirs

In a three-level sequence stratigraphic framework, according to target exploration series of strata, a sedimentary facies map, major effective hydrocarbon source rock distribution map, tectonic setting map and exploration level map for target formations of various assemblages of reservoir formation are organized and compiled. Our application of overlaying the four maps for comprehensive analyses leads to the classification and evaluation of favorable litho-stratigraphic oil and gas reservoir accumulation zones.