Oil-gas reservoir in the Mesozoic strata in the Chaoshan depression,northern South China Sea:a new insight from long off set seismic data

The Chaoshan depression,a Mesozoic basin in the Dongsha sea area,northern South China Sea,is characterized by well-preserved Mesozoic strata,being good conditions for oil-gas preservation,promising good prospects for oil-gas exploration.However,breakthrough in oil-gas exploration in the Mesozoic strata has not been achieved due to less seismic surveys.New long-off set seismic data were processed that acquired with dense grid with single source and single cable.In addition,the data were processed with 3D imaging method and fi ner processing was performed to highlight the target strata.Combining the new imaging result and other geological information,we conducted integrated interpretation and proposed an exploratory well A-1-1 for potential hydrocarbon.The result provides a reliable basis for achieving breakthroughs in oil and gas exploration in the Mesozoic strata in the northern South China Sea.

Characteristics and Hydrocarbon Potential of Mesozoic Strata in Eastern Pearl River Mouth Basin, Northern South China Sea

The well LF35-1-1 in the eastern Pearl River Mouth basin （PRMB） of the northern South China Sea revealed unmetamorphosed Middle-Late Jurassic neritic-bathyal sediments and Cretaceous fluvial-lacustrine sediments. Three tectonic movements were identified in Late Jurassic to Early Cretaceous, late Early Cretaceous, and Late Cretaceous to Paleocene, respectively. The Late Jurassic marine facies mainly contain the hydrocarbon source and reservoir-seal assemblages, providing a main exploration target.

Seismic Acquisition Parameters Analysis for Deep Weak Reflectors in the South Yellow Sea

The Mesozoic-Paleozoic marine residual basin in the South Yellow Sea(SYS) is a significant deep potential hydrocarbon reservoir. However, the imaging of the deep prospecting target is quite challenging due to the specific seismic-geological conditions. In the Central and Wunansha Uplifts, the penetration of the seismic wavefield is limited by the shallow high-velocity layers(HVLs) and the weak reflections in the deep carbonate rocks. With the conventional marine seismic acquisition technique, the deep weak reflection is difficult to image and identify. In this paper, we could confirm through numerical simulation that the combination of multi-level air-gun array and extended cable used in the seismic acquisition is crucial for improving the imaging quality. Based on the velocity model derived from the geological interpretation, we performed two-dimensional finite difference forward modeling. The numerical simulation results show that the use of the multi-level air-gun array can enhance low-frequency energy and that the wide-angle reflection received at far offsets of the extended cable has a higher signal-to-noise ratio(SNR) and higher energy. Therefore, we have demonstrated that the unconventional wide-angle seismic acquisition technique mentioned above could overcome the difficulty in imaging the deep weak reflectors of the SYS, and it may be useful for the design of practical seismic acquisition schemes in this region.

Uplift and evolution of Helan Mountain

The Helan Mountain lies in the northwest margin of Ordos Basin and its uplift periods have close relations with the tectonic feature and evolution of the basin.There are many views on the uplift time of Helan Mountain,which is Late Triassic and Late Jurassic.It is concluded by the present strata,magmatic rock and hot fluid distribution that the Helan Mountain does not uplift in Late Triassic to Middle Jurassic but after Middle Jurassic.Through the research of the sedimentary strata and deposit rate in Yinchuan Graben which is near to the Helan Mountain,it is proved that the Helan Mountain uplifts in Eocene with a huge scale and in Pliocene with a rapid speed.The fission track analysis of apatite and zircon can be used to determine the precise uplift time of Helan Mountain,which shows that four stages of uplifting or cooling:Late Jurassic to the early stage of Early Cretaceous,mid-late stage of Early Cretaceous,Late Cretaceous and since Eocene.During the later two stages the uplift is most apparent and the mid-late stage of Early Cretaceous is a regional cooling course.Together with several analysis ways,it is considered that the earliest time of Helan Mountain uplift is Late Jurassic with a limited scale and that Late Cretaceous uplift is corresponding to the whole uplift of Ordos Basin,extensive uplift happened in Eocene and rapid uplift in Pliocene.