Abstract:

In the Paleozoic buried hills of the Jiyang Depression, the complex middle-lower section between the positive structural units has become the main exploration focus after the initial prospecting of the residual hill and peripheral buried hills. The eastern part of the Zhuanghai region is close to the Tanlu fault zone and has been profoundly impacted by its multistage tectonic movements. The structural style of the Zhuanghai Paleozoic buried hill is very complex, and deep burial can result in low accuracy in the early seismic data, causing ambiguous structural characterization of the zhuanghai area thus greatly hindering the exploration progress. In this paper, we discussed the complex structural characteristics and formation mechanism of the Paleozoic buried hills of the Zhuanghai area, using high precision 3D seismic and drilling data and regional stress field test results. Our results showed that a set of paleozoic stable marine and transitional sedimentary rock series developed in the Zhuanghai area, where Cambrian, Ordovician, Carboniferous and Permian series developed successively from bottom to top. There are four groups of different types of Paleozoic faults in the Zhuanghai area, grouped as the NW, SN, NE and EW-trending faults. The five NW-trending faults are high angle normal and reverse faults; the three NS-trending faults are strike-slip faults; the NE-trending faults, which cut through the NW-trending fault group, are normal faults; and the EW faults, crossing the NW fault group, are mainly the southern and northern boundary faults of the Zhuanghai buried hills.

The fault development can be divided into four periods: the first period in the Indosinian when reverse faults formed by Paleozoic compression; the second period from the Late Jurassic to Early Cretaceous when normal faults formed by structural reversions through the Mesozoic-Paleozoic era; the third period in the Late Cretaceous when thrust faults formed by Mesozoic-Paleozoic compression; and the fourth period in the Eocene when normal faults formed by strike slip and extension movements through the Paleozoic-Mesozoic-Paleogene-Neogene period. Intersecting and intercutting faults of different trendings formed a “chessboard” type complex structure. The Paleozoic top surface resembled a W-E trending anticline and can be divided from west to east into four rows of buried hills. The complex evolutionary process is responsible for the disparity in the reserved strata and for the structural types of different rows of buried hills. Formation mechanism study showed that the Late Triassic to Eocene Paleozoic Zhuanghai strata underwent four stages of evolution: compression, extension, compression, and strike-slip, under the control of the reciprocating strike-slip stress field of the Tanlu fault. At the end of the Triassic, three NW-trending thrust fold belts formed under the control of the NE-SW-trending compression by the sinistral strike-slip movement of the Tanlu fault in the Zhuanghai area. From the Late Jurassic to Early Cretaceous, the thrust faults reversed unevenly, resulting in the NW-trending alternation of normal and thrust faults. In this period, four rows of NW-trending buried hills were formed. The Zhuanghai area of the Late Cretaceous experienced a 2^nd NWW-SEE-trending compression, under a conversional stress field of the Tanlu fault in a sinistral to dextral strike slip movement. From the second to the fourth row of the buried hills, the strata were napped from east to west and the Paleozoic structures were reconstructed again to establish the basic structural pattern of the Paleozoic. In the Eocene, the NS-trending faults, NE-trending strike slip faults and EW-trending adjustment faults were formed by the dextral strike slip movement of the Tanlu fault, intersecting the early formed NW-trending faults and a south to north downward tilting movement occurred in the Zhuanghai area at the same time to form the present-day complex structural styles. The structural evolution controlled the disparities in the buried hill structures as well as reservoir forming conditions in the Paleozoic Zhuanghai area. The stratum gradually thinned from the east to the seriously eroded west. The first row of buried hills only retained Cambrian strata, while Ordovician strata reserved in the three rows to the east gradually thickened. The first row of buried hill developed fault block traps of Cambrian strata; the second row developed anticline traps of Ordovician and Cambrian strata; and the third and fourth rows developed fault block traps of Ordovician and Cambrian strata. The trap types changed from fault block on both sides to fold in the middle, resulting in better reservoirs. The second row of Ordovician thrusting fold hills are most favorable for forming reservoirs.

Key words: structure characteristics, stress field, tilting movement, Tanlu fault, Zhuanghai area, Jiyang Depression