Abstract:

The Weixinan Depression is an important oil and gas accumulation area in the Beibu Gulf Basin of the South China Sea, and turbidite sand body is an important reservoir contributing geological body showing great oil and gas exploration potential. With the exploration and production constantly advancing, it is difficult to identify a true turbid reservoir by conventional geophysical techniques due to severe overlapping of sandstone and mudstone’s seismic characteristics. For this reason, considering the association of strong amplitude reflection with formation of turbidite sand body, we analyzed the two main causes of the abnormal strong amplitude reflection in the non-reservoir formation, i.e., easy production of strong amplitude reflection by interbedding of thin sandstone and mudstone, and severe interference in strong amplitude response of low velocity mudstone from indistinguishable responses of sweet reservoir and surrounding rock. On this basis, we put forward a new technology for reservoir identification based on energy half-time. Firstly, after the isochronous stratigraphic reflection interface is determined, and a fine isochronous stratigraphic framework is established, the fifth-order sequence is examined as a set of equivalent bodies. Because the densities of sandstone and mudstone are relatively stable, it is established that the slight amplitude variation inside the abnormal body mainly reflects the change of velocity structure. That is, the change of velocity structure is key to distinguish true reservoirs from false ones. Secondly, a wedge-shaped model of sandstone and mudstone interbedding is established, which demonstrates that different lithologic compositions correspond to different velocity structures, and different velocity structures cause the change of the attribute value when the energy half-time is changed. That is, the change of velocity structure in thin sand-mud interbedding can be effectively described by the property of energy half-time. Thirdly, based on the fine velocity analysis of the study area, slight amplitude variation inside the anomaly body can be interpreted by forward modeling according to three combination schemes: same lithology and physical property, same lithology but different physical properties, and different lithologies and physical properties. Hence, the main cause of the impedance change can be determined, and the possible composite lithology of the abnormal body can be predicted synthetically to reduce prediction ambiguity. Applying this technology to the case exploration, the results of lithology prediction agrees with the drilling findings, which strongly supports the exploration and production practice in the Weixinan Depression.

Key words: seismic characteristics overlapping, turbidite sand body, amplitude micro-variation, velocity structure, energy half-time, forward modeling, lithology combination prediction, Weixinan Depression