258 / 2019-06-30 23:04:21

Sedimentary characteristics of nearshore subaqueous fans in the steep slope of rift basin based on FMI image ——Taking the upper Es4 Formation in Yong'an area of Dongying Sag, Bohai Bay Basin, China as

rift basin,nearshore subaqueous fan,image logging,sedimentary microfacies

A large-scale nearshore subaqueous fan is developed in the upper part of the fourth member of the Eocene Shahejie Formation in the northern steep slope of the Dongying Sag, China. It has multi-source, close-range and rapid accumulation characteristics, and its compositional maturity and structural maturity are low, and the lithology changes greatly, the lateral change is fast. Based on core observation and description, this paper uses the core data to scale the FMI image, analyzes the lithofacies facies types and characteristics, and identifies 4 major categories and 7 subtypes, including conglomerate, pebbly sandstone, sandstone and mudstones. They have large particle size spans, mixed particles of different particle sizes are common, and near source rapid accumulation characteristics are obvious. Single well phase analysis was carried out by different types of lithofacies combined with logging curves: (1) Inner Fan main channel microfacies. It is mainly developed with multiple sets of granule support in coarse conglomerate and heterogeneous support in coarse conglomerate, fine conglomerate and pebbly sandstone’s superimposed. The bottom is developed with scour and filling structure. Gravel is mixed with mud and sand is common and the sorting is poor. Gravel mostly shows angular to sub-angular, which sometimes are piled at high angles. It shows bright or dark spots on FMI images, with low acoustic time and high resistivity characteristics, and developed massive bedding . (2) The microfacies of the braid channel in the Middle Fan. It is mainly composed of conglomerate, fine conglomerate, pebbly sandstone, high-resistance gray sandstone and argillaceous sandstone, developed with granular sequence bedding. The FMI image shows multiple speckle patterns to the highlights model of blocky gravel and to the superposition of the layered pattern sequence. A large number of scour-fill structures are visible at the bottom of the channel, and the gravel is sub-angular to sub-circular; the acoustic time logging value is medium-low and the resistivity is medium-low; (3) The microfacies between the channel in the Middle Fan. The lithology between the channel is gradually transformed into fine sandstone, siltstone or pebbly fine sandstone with thin layer of mudstone, and the FMI image is interbeded light and dark. (4) The microfacies of the front of the channel in the Middle Fan. Gravel gradually decreases, and it develops into a combination of sandstone, sand-mud interbed and mudstone. Gradually, the mudstone layer becomes thick, the horizontal bedding is formed in the mudstone, and sandwiched with a thin layer of sand; the FMI image shows the band-like distribution of interbeded light and dark. The logging curve shows the characteristics of high acoustic time and low resistivity.