Sequence Stratigraphy Analysis and Lithologic Trap Model of Eocene, Laizhou Bay Depression, Bohai Bay Basin, Eastern China
The Laizhou bay depression is an abundant hydrocarbon-generating fault basin. Oil mainly come from the middle shahejie formation (E2S3). The resource rock in this formation is about 400 meter thick all over the sag. There are a large scale turbidite fan and a set of braided delta system in this formation, but it is difficult to identify their sand bodies using seismic data due to the low resolution of seismic in deep layer. To identify and describe these sand bodies accurately, sequence stratigraghy framework was established to help the analysis of seismic facies. First, regional third-order sequence stratigraphy framework was established in the middle E2S3 in Laizhou Bay depression based on the 3D seismic data and the well logging data. In the low stand system tract (LST) of the middle E3S3, a turbidite fan was recognized and dividided into 3 subfacies: the inner fan subfacies, the middle fan subfacies and the outer fan subfacies according to seismic facies. The middle fan subfacies is mainly superposed by sandrock and form the fine reservoir. Second, the fourth-order sequence stratigraphy framework was established in the high stand system tract (HST) of the middle E3S3. The braided delta system was divided into 6 fourth-order sequences corresponding to 6 phases braided deltas. According to the seismic facies analysis in 6 fourth-order sequences, 3 types of sand body were identified: the braided delta foreset sand body, the slope-shifting turbidite fan which was the product of braided delta erosion and resedimentation, the slump turbidite fan which was the product of braided delta foreset sand slumping. These 2 types of fan are all enveloped by thick resource rock. The braided delta foreset sands are covered by braided delta plain, which is high sand-shale ratio formation. With the braided delta prograding forwards, the sand-shale ratio of the cover formation decrease gradually and the reconstruction of lake water increase gradually. The shaliness and effectiveness of the cover formation increased gradually. The braided delta foreset sands of number 1-3 phases are ineffective traps, while the number 4-6 phases are effective traps. The KL10-1 oilfield with over 100,000,000 cubic meter proven oil reserves explores only the north high of 4th and 5th phases of the braided delta foreset sands. The south high of 4th and 5th phase, the 6th phase of the braided delta foreset sands and 3 types of fan described have huge exploration potential.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California