Jurassic sequence framework and sedimentary system distribution in the Wudun Sag, Dunhuang Basin

doi:10.13745/j.esf.sf.2020.5.17

Abstract

Abstract:

The Dunhuang Basin is a Mesozoic rift basin with relatively high exploration potential, and is in the early stage of petroleum exploration. As it is known, the basis of petroleum exploration is building the sequence framework and obtaining the sedimentary system distribution. Using the outcrop, core, lithology, well-logging and seismic data, and taking into account of various geological factors, such as tectonic settings and sediment supplies, etc., we identified the sedimentary facies types and determined the sedimentary distribution in the Wudun Sag, Dunhuang Basin. The findings show that the main Middle-Lower Jurassic strata in the study area can be divided into two third-order sequences SQI and SQII. Roughly, SQI corresponds to the Dashankou Formation, and SQII to the 1^st member of the Zhongjiangou Formation. Due to the movement of boundary faults, the Wudun Sag is a half graben-like sag with steep slope in the south and gentle slope in the north, so the sequence distribution thins out from south to north. The material sources in the Dunhuang Basin primarily originated from the Bei, Altun-Qilian and Sanwei Mountains. The sedimentary distribution in the Wudun Sag was controlled by the source supplies from the north, south and east. The Middle-Lower Jurassic strata of the Wudun Sag developed three types of facies: braided river delta, fan delta and lakes. The sedimentary type for Wells XC1 and Dun1 is the braided delta front. Fan delta without drilling wells can be identified by outcrops and seismic data. Lacustrine consists of shore-shallow lake (including lake mud and beach and bar microfacies) and sublacustrine fan (including sand and mud flow deposits). The developmental scale and distribution mechanism of sandbodies were primarily subjected to tectonic and provenance controls, and the sedimentary distribution of different sequences are roughly the same. Continued accumulation of terrigenous detrital rocks in the north and northeast left behind braided delta sediments on the gentle slopes, while, affected by material sources from the Sanwei Mountain, the fan delta sedimentary system developed in the south, and littoral-shallow lacustrine sediments deposited in the central depression.

Key words: Dunhuang Basin, Wudun Sag, Middle-Lower Jurassic, provenance, sedimentary types, sedimentary distribution

CLC Number:

DONG Yanlei, ZHU Xiaomin, WEI Minpeng, LI Shunli, ZHAO Ruixing, JIA Tianpeng, GUAN Weifeng, ZHANG Yaxiong. Jurassic sequence framework and sedimentary system distribution in the Wudun Sag, Dunhuang Basin[J]. Earth Science Frontiers, 2021, 28(1): 177-189.

Figures/Tables 14

Fig.1 (a) A sketch map of the structural units in the Dunhuang Basin. (b) Outcrop locations in the Wudun Sag. (c) Diagram showing the extend of exploration in the Wudun Sag.

Fig.2 The structural characteristics of the Jurassic strata in the Dunhuang Basin. Adapted from [9].

Table 1 Comparison of the Middle-Lower Jurassic sequence stratigraphic and lithostratigraphic units in the Wudun Sag, Dunhuang Basin

Fig.3 Isopach map showing tract thickness of different Middle-Lower Jurassic systems in the Wudun Sag, Dunhuang Basin

Fig.4 Predicating the direction of sediment supplies according to the residual Bouguer gravity anomaly map

Fig.5 Sediment supplies and sedimentary sequence of field outcrops around the Wudun Sag, Dunhuang Basin

Fig.6 Characteristics of northern and southern palaeoflow directions for the Duobagou outcrop around the Wudun Sag, Dunhuang Basin

Fig.7 Predicating the direction of sediment supply according to the forest seismic facies and channels (See Fig.1b for seismic cross-section and well locations)

Fig.8 Sedimentary characteristics of subaqueous distributary and interdistributary channels of the braided-delta front in the Wudun Sag, Dunhuang Basin

Fig.9 Sedimentary characteristics of point bar of the braided-delta front in the Wudun Sag, Dunhuang Basin

Fig.10 Lithologic association characteristics of the Jurassic fan-delta sedimentary outcrop (Nanhu) around the Wudun Sag, Dunhuang Basin

Fig.11 Lithologic association characteristics and sedimentary sequence of the Jurassic sublacustrine fan in the Wudun Sag, Dunhuang Basin(Well Dun1)

Fig.12 Sequence and sedimentary characteristics of Middle-Lower Jurassic strata in joint Wells XC1 and Dun1 in the Wudun Sag, Dunhuang Basin (See Fig.1c for cross-section location)

Fig.13 Sedimentary characteristics of SQI-SQII sequence in the Wudun Sag, Dunhuang Basin

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