Tectono-Sequence Stratigraphic Analysis on Paleogene Shahejie Formation in the Banqiao Sub-Basin, Eastern China

Tectono-sequence stratigraphic analysis on Paleogene Shahejie Formation

in the Banqiao sub-basin, Eastern China

by Yuguang Hou a,*, Sheng He a, Jun’e Ni b, Bingjie Wang a,c

INTRODUCTION

The Banqiao sub-basin is a standout among the most imperative oil-and gas areas in the Huanghua dejection of the Bohai Bay basin. In the Banqiao sub-basin, its sequence types, the depositional systems and mainly the distribution of sand bodies are controlled by tectonics. Tectonics plays a really important role in this matter. Besides that, the Banqiao sub-basin has been categorize to mature phase under petroleum exploration. In addition, referring to a model of “precise structure exploration”, various structural traps have been discovered and locating new oil reserves has become harder. A tectono-stratigraphic approach may enhance oil exploration in this mature phase of oil exploration.

BASIN DEVELOPMENT

The location of Banqiao sub-basin is at central-north Huanghua depression, Bohai Bay basin, Eastern China. The sub-basin was developed as an asymmetrical half-graben along a major NE- trending basin-margin fault, the Cangdong fault. Bounding the basin to northwest is the Cangxian uplift, to the southeast the Beidagang buried hill structural zone and the Qikou sub-basin buried, the northeast the Haihe faults and the Tanggu buried hill structural zone and the south-west the Shenqingzhuang buried hill structural zone. Furthermore, north faults steep slopes belt, the middle depressed belt and the south Banqiao faulted structure belt are the sub-structures units of the Banqiao sub-basin. This basin contents 7km thickness worth of Tertiary sediments. Besides that, with deposition of Shahejie and Dongying formations, the main geological age of subsidence of this basin is Palaeogene. Sediments of the Palaeogene age are much thicker than sediments of Neogene age. These formations are confined to the half graben and deposits in fluvial-lacustrine conditions. The main source rock horizons and the key reservoir beds all the contents of the Shahejie formation. This formation can be subdivided from oldest to youngest according to its lithology and fossils assemblages.

BASIN FORMATION

Located on east coast of China is the Bohai Bay basin which is one of the prime petroleum basin. It was formed at North China craton with a complex evolution history. This basin created as a back-arc basin in Mesozoic and advanced into an intracratonic rift basin in Cenozoic. Next, fault activity, basin subsidence and altered paleogeography are the cause of occasionally extension during the syn-rift stage. These three process or activity controls depositional systems, sequence patterns and sand-body accumulations distribution.

Bohai Bay basin consists of a sub-unit known as the Huanghua depression. Its Cenozoic basic development amid the incorporated a Paleogene syn-rift phase of and a Neogene post-rift subsidence phase of. Syn-rift stage can be subdivided to initial rift stage, main rift I stage, main rift II stage and late rift stage. According to subsidence rate and strength of tectonic activity, the Paleogene development of Banqiao sub-basin is categorize into three phases which are first rifting phase from Es3 to Es2 (Eocene), the second rifting phase of Es1 (Oligocene) and third rifting phase of Ed (Oligocene). Then, at the time of the main rift stage, the Shahejie Formation was deposited. At this time, basin subsided was constant at the same time uplift of basin margins happened bringing about expanded differential palaeotopography. As the results, the rate of sediment supply which are from the Cangxian uplift and the Beidagang buried hill structural zone became very high. [pic 1]

Besides that, the Shahejie Formation and the Dongying Formation can categorize into three second-order sequence and eleven third-order sequences. In each 3rd order sequence, three type of system tracts are found which are low-stand system tract, transgressive system tract and high-stand system tract. For every 2nd order sequence relating with one phase of occasionally rifting stand for depositional filling of this rifting phase. Moreover, it is the outcome of basin developed from beginning low subsidence to maximum subsidence, reducing subsidence and inactive stage.

Furthermore, units of Es3 to Es1 are subdivided from the Shahejie Formation. Es3 to Es2 is related to the regional main rift stage I. At this time, large scale low-stand fan and incised valley deposits makes up the low-stand system tract. From this, we know that the tectonic activity was strong topography in the provenance areas was high, and relief on the basin margin was substantial. The supreme sediments sources are the Cangxian uplift and the Beidagang buried hill. In those system tracts, deltaic and gravity flow deposits are often found. In addition, on the fault margin mostly fan deltas forms and on the ramp margin mostly braided-deltas. Structurally controlled stratigraphic framework are the results of subsidence and uplift due to activity of Cangdong fault. Inside a 3rd order sequence, deltas responded to accommodation within the main rift stage I from initial low subsidence to maximum subsidence, reducing subsidence, and finally an inactive stage.

Es1 related to the regional main rift stage II. Following, increase in total subsidence rate of the Banqiao sub-basin resulting the stratigraphic framework pattern of deposition changed. Small-scale low-stand fans and wedges with fan-deltas are large components of the low-stand system tract that forms on the fault margin. A part of it stands for increased transgressive system tract. Not only that, the Cangxian uplift is still an important origin of sediment source. Due to strong subsidence, the Beidagang buried hill sediment supply reduces. Next, amount of braided-delta and fan-delta of low-stand system tract reduced dramatically on the southern ramp margin. Thus, longshore bar deposits and carbonate beaches or barriers in high-stand system tract forms. [pic 2]