Abstract:

The Qiongdongnan Basin is located in the northwestern part of the continental shelf in the northern part of the South China Sea, which is a Cenozoic highly-overpressured, transformed, and extended basin with abundant oil and gas resources. The Qiongdongnan Basin is also a typical offshore HTHP basin in China, and is one of the world’s top 3 offshore HTHP areas, with the highest reservoir temperature of over 240 ℃ and the highest formation pressure coefficient of 2.3, which makes it one of the most difficult areas for domestic and international drilling operations. The complex genesis mechanism of abnormal high pressure in the Qiongdongnan Basin of the South China Sea results in great difficulty and low accuracy of pre-drilling pressure prediction, which seriously compromises drilling safety. Block 64/07 of Lingshui structure in Qiongdongnan Basin was taken as the research object. Through analysis of tectonic evolution and loading/unloading mechanisms, the coupling genesis mechanisms of abnormal high pressure in this block were revealed, a multi-mechanism coupling pressure prediction method was specifically established, and formation pressure profiles of drilled wells and a 3D regional formation pressure model were constructed, systematically analyzing the vertical and horizontal pressure systems of this block. The Qiongdongnan Basin has experienced many rounds of tectonic evolution since the Paleocene, forming a huge thickness of Paleocene-Neocene and Quaternary sediments. From the perspective of the depositional history of the Qiongdongnan Basin, the sedimentation rate of the whole basin is relatively high, with two distinct peaks. The first peak occurred in the fracture to fracture-argument period, with a subsidence rate exceeding 600 m/Ma. The second appeared during the post-fracture rapid subsidence period, with the Yinggehai Formation reaching 600~1 700 m/Ma. Both rates exceeded the 150 m/Ma threshold for overpressure, and these periods were prone to the formation of anomalous high pressures with undercompaction mechanisms. These processes were the basis of the overpressure prevalent across the entire basin, and were manifested as loading-type anomalous high pressures. Meanwhile, the FS fracture zone in this region had the ability to transmit oil and gas, forming a two-way hydrocarbon supply pattern between the main depression and the secondary depression in western Lingshui. The hydrocarbons had two large-scale hydrocarbon discharge periods (15 Ma and 5 Ma), and the Meishan~Sanya Formation underwent prolonged effective charging, creating conditions for high pressure caused by fluid charging, which was the most probable auxiliary cause of anomalous high-pressure formation in this region. Using the logging data of the drilled wells, the rock density-sound wave velocity crossplot of the stratified sections was established, and the Meishan Formation showed a composite mechanism of loading and unloading. Combined with the tectonic evolution history, it could be inferred that the anomalously high pressure of Yinggehai and Huangliu Formations was attributed to the undercompaction mechanisms, and the anomalously high pressure of Meishan Formation resulted from the coupling mechanism of undercompaction and fluid filling. To address the high-pressure coupling genesis mechanism of this block, this study developed a multi-mechanism coupling pressure prediction method. The upper Yinggehai Formation and Huangliu Formation under-compacted strata were evaluated using the conventional Eaton method, and the lower Meishan Formation and Sanya Formation stratigraphic over-pressure zones were analyzed using the multi-parameter effective stress method, thereby overcoming the limitations of single-method prediction and improving prediction accuracy. Based on this method, a high-precision three-dimensional stratigraphic pressure body was constructed in Lingshui tectonic block 64/07, revealing the vertical and horizontal distribution patterns of stratigraphic pressure in this block. The results showed that the high pressure in formations above Huangliu Formation in this area was caused by undercompaction mechanism, while below Meishan Formation it was caused by the coupling mechanism of undercompaction and fluid charging. Vertically, pressure began to increase in the middle and lower part of Yinggehai Formation, Huangliu Formation served as the pressure transition zone, and Meishan and Sanya Formations entered the overpressure zone, with the highest pressure coefficient reaching 2.10. Horizontally, it generally presented the characteristics of “low in the west and high in the east, low in the north and high in the south”. These research findings were applied to well L5-1. The core of this method lay in addressing the coupling genesis of formation pressure in the Meishan Formation by establishing a multi-parameter effective stress method. Parameters such as shale content, porosity, and effective stress of the lower high-pressure layer were introduced, avoiding the difficult problem of determining original sedimentary loading and subsequent unloading, thereby achieving accurate prediction of reservoir high pressure. During the drilling process of well L5-1, a combination of pre-drilling pressure prediction, logging monitoring, logging constraints on seismic layer velocity and MDT pressure correction were used to follow up and monitor the trend of pressure change throughout the entire process in real time. The entire pressure monitoring process could be divided into four stages: (1) determining the starting pressure layer at the pre-drilling design stage as the bottom of Ledong Formation and the top area of Yinggehai Formation; (2) adjusting the starting pressure depth from 2 100 m to 1 850 m based on the three-opening single-root gas pressure and the pressure monitoring profile with drilling; (3) using the four-opening electroacoustic constraints to recalibrate the extracted layer velocity, combined with the MDT pressure measurements, to correct the parameters of the prediction model; (4) conducting five-opening monitoring and the DST test after completion of drilling to complete the real drilling pressure evaluation. Using this method, the average accuracy of pre-drilling formation pressure prediction was 87.1%. During actual drilling, based on logging and test data, prediction results were timely corrected, and the prediction accuracy of lower formation pressure was improved to 98.8%, meeting the requirements for drilling design and field construction.

Key words: Qiongdongnan Basin, Lingshui structure, abnormal high pressure, mechanism of abnormal pressure, multi-parameter effective stress method