Sedimentary characteristics of large‐scale lacustrine beach‐bars and their formation in the Eocene Boxing Sag of Bohai Bay Basin, East China

Abstract

The beach‐bar reservoir play has become an important exploration target within the Bohai Bay Basin, especially in the Boxing Sag within the Dongying Depression, where a large‐scale lacustrine beach‐bar oil pool has been discovered recently. The sedimentary characteristics, distribution patterns and formation mechanisms of beach‐bar sand bodies in the upper fourth member of the Eocene Shahejie Formation (Es4s) in the Boxing Sag were studied in detail based on seismic, well log data and core data. The Es4s in the Boxing Sag is composed of a third‐order sequence consisting of three systems tracts, i.e. a lowstand systems tract, a transgressive systems tract and a highstand systems tract. Beach‐bar sand bodies were deposited widely in the basin during the lowstand systems tract period. The sandy beach‐bars are characterized by siltstones, fine‐grained silty sandstones interbedded with thin mudstone units. The presence of well‐developed sedimentary structures, such as swash bedding, parting lineation, parallel bedding, ellipsoidal mud clasts, ripples, terrestrial plant debris and vertical burrows, suggests that beach‐bars were deposited in a relatively shallow water environment under the influence of strong hydrodynamics. Laterally, the sandy beach facies occurred as a more continuous sheet‐like body around the sandy bar in most parts of the sag. Stratigraphically, beach‐bars were distributed mainly in the lowstand systems tract and they were less well‐developed in the transgressive systems tract and highstand systems tract. Several factors were probably responsible for the occurrence of the large‐scale beach‐bars during the lowstand systems tract period, including: (i) a gentle palaeoslope and relatively weak structural activities; (ii) a shallow‐water condition with a strong hydrodynamic environment; (iii) high‐frequency oscillations of the lake level; and (iv) an abundant terrigenous clastic feeding system with multiple‐point and linear sediment sources.

A method to define the palaeowind strength from lacustrine parameters

“…(ii) If the gravel beach ridges have undergone significant mechanical compaction, then these thicknesses are decompacted (Sclater & Christie, ). (iii) The palaeowater depth is estimated from the rock record (Li et al ., ; Immenhauser, ; Jiang et al ., ; Pochat & Van Den Driessche, ; Su et al ., ) to obtain the palaeowater depth ( d ). (iv) The palaeowind direction is reconstructed based on the available rock or sediment record (Orme & Orme, ; Allen, ; Krist & Schaetzl, ; Miao et al ., ; Pochat et al ., ; Jewell, ; Jiang et al ., ).…”

“…The southern hinged margin was occupied by an extensive shore‐shallow lake system with a low bathymetric gradient (Jiang et al ., ) where large‐scale beach‐bar systems developed (Li et al ., ; Jiang et al ., , ). The axial margin was dominated by fluvial deltas (Jiang et al ., ; Feng et al ., ). These depositional systems are similar to previously reported half‐graben lacustrine depositional models (Soreghan & Cohen, ).…”

“…(A) General Cenozoic stratigraphy of the Dongying Depression (modified from Jiang et al ., ). The ages were determined from dating of volcanic rocks (Chen & Peng, ; SLOF, ; Li et al ., ; Ye et al ., ; Feng, ; Pan et al ., ).…”

“…The northern faulted margin depositional systems are proximal systems that include fan deltas and sublacustrine fans associated with storm deposits and turbidites (Li et al ., ; Liu & Wang, ; Feng et al ., ; Wang et al ., , ); the bathymetric gradient was great there. The southern hinged margin was occupied by an extensive shore‐shallow lake system with a low bathymetric gradient (Jiang et al ., ) where large‐scale beach‐bar systems developed (Li et al ., ; Jiang et al ., , ). The axial margin was dominated by fluvial deltas (Jiang et al ., ; Feng et al ., ).…”

Quantitative evaluation of the reservoir potential and controlling factors of semi-deep lacustrine tempestites in the Eocene Lijin Sag of the Bohai Bay Basin, East China

“…(A) General Cenozoic stratigraphy with the stratigraphic age of the Dongying Depression in the Bohai Bay Basin (modified from Jiang et al, 2011). (B) Detailed Es4 1 stratigraphy of the Dongying Depression (modified from Li et al, 2008;Tian and Jiang, 2009), which shows the lithology section, sequence stratigraphy scheme, and sedimentary environment interpretation.…”