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Kefei, Shen (BGP, CNPC) | Qinghong, Zhan (BGP, CNPC) | Lin, Sun (BGP, CNPC) | Gongxun, Lv (CNPC international (Turkmenistan)) | Henian, Liu (CNPC international (Turkmenistan)) | Lei, Wu (CNPC international (Turkmenistan)) | Yequan, Chen (CNPC international (Turkmenistan))
Summary: Due to the harm that abnormal pressure can cause, geopressure prediction is always necessary for the safe exploitation of oil resources. Because of the far-ranging overpressure in the carbonated rock below the salt in the Turkmenabat terrace of Turkmenistan, geopressure prediction was investigated by using seismic data, logging and related geological data. By analyzing this data and combining the observations with the calculation method of pore pressure prediction, we come up with an empirical method for predicting geopressures. This method provided the theoretical foundation for geopressure prediction for carbonated rock below salt in the Turkmenabat terrace. Introduction: Abnormal pressure has been found in many petroliferous basins. Geopressure is distributed in most oil fields of the world (Japsen, 1994). Because of the serious hazards caused by geopressure during exploratory development, differing methods of geopressure prediction has been explored for many years. In 1968, Pennebaker published the paper entitled “Seismic data depth magnitude of abnormal pressures” in World Oil. His paper provided an idea for geopressure prediction by geophysical method (Pennebaker, 1968). With the improvement of the geophysical technique in recent years, this idea was gradually applied to geopressure prediction, and benefited development of many oil fields in the world to some extent. There are two methods for geopressure prediction by geophysical technique: prediction by well logging and prediction by seismic data. Although the former has its high accuracy, it is difficult to predict on a large scale given the limited distribution of wells. The latter is not confined by original data because seismic data covered all the oil fields in the world. The seismic reflection stratum manifested not only the lateral feature of the structure, but also the velocity and the wave impedance, making it possible to obtain the distribution of the pore pressure. Now prediction by seismic data has been the primary method for geopressure prediction before drilling all over the world (Zhang et al., 2005). The rock frame stress is in close connection with its compaction, which is greatly affected by its density, and the change of density will be eventually shown on its interval velocity field (Zu et al., 2004). Since geopressure prediction by seismic data mainly depends on the change of the interval velocity, the interval velocity is ideal data for geopressure prediction. Velocity can be obtained from the seismic velocity spectrum, which is created from the common reflection-point gather (CRP gather). The relationship between velocity and pressure may be set up by related logging data. To better study geopressure prediction for carbonated rock below salt, we have selected part of 3D seismic data from the middle of Amudarin block in Turkmenistan. The reservoir in this area is a deeply-buried carbonate reef below salt of Jurassic. Due to the limitation of the engineering, it is the potential area of exploratory development in Turkmenistan. Although 2D surveys have covered the whole area since the acquisition started from 1970s, it is difficult to distinguish the reflector of the target due to its poor quality.