Comprehensive Reservoir Prediction in an Offshore Sandstone and Conglomerate Fan Delta Reservoir: A Case Study of Paleogene Shahejie Formation in Jinzhou 25-1 Oilfield, Bohai Bay Basin

Jinzhou 25-1 Oilfield is located in the northern portion of the generally north-south trending Liaoxi Uplift in the northern Bohai Bay Basin. It is a giant oilfield that has proven reserves of more than 400 million barrels of... [ view full abstract ]

Jinzhou 25-1 Oilfield is located in the northern portion of the generally north-south trending Liaoxi Uplift in the northern Bohai Bay Basin. It is a giant oilfield that has proven reserves of more than 400 million barrels of oil and gas. The main reservoir formation (Shahejie) were deposited in a fan delta of Paleogene. Fan delta reservoir mainly forms in active tectonic regions, delta distributary channels moved laterally and overlapped frequently, resulting in a series of vertically stacked sand bodies. Shahejie Formation has good porosity and permeability, however, laterally complex sandstones and conglomerates create the need for a precise interwell estimation of reservoir distribution. The high cost and risk in offshore drilling makes the traditional reservoir prediction based on dense well network unable to depict its complex facies distribution. In this paper, geology-geophysics comprehensive reservoir prediction is carried out to describe the favorable digenetic facies distribution of Jinzhou 25-1 Oilfield in Bohai Bay Basin. Firstly, lithofacies were identified through the cores and well-log data, including sedimentary facies type, sedimentary structure and stacking patterns. Depositional model for the study area is the basis of sequential studies. Secondly, based on the high resolution sequence stratigraphy framework, the scope of fan delta were identified with the distribution of seismic facies from the adjacent lake deposition. Thirdly, with adequate well control and good ties to 3D seismic data, attribute expression of different reservoir facies were used to analyze the distribution of favorable sedimentary facies zone. Finally, guided by seismic sedimentology, the seismic slices of each sedimentary cycle were extracted within the favorable sedimentary facies zone targeted above. The phases of thick-bedded fan delta distributary sand bodies can be divided. Then, the vertical distribution of fan delta distributary sand bodies were predicted with 3-D wave impedance volume derived from log constrained inversion. The results showed that the most developed sand body was distributary channel. RMS, frequent amplitude and sweet spots were optimized to predict the distribution of reservoir. The study indicated that reservoir prediction built with geology-geophysics reflects the sedimentary facies space distribution in fan delta more accurately. The insight of reservoir facies distribution makes it possible to improve the efficiency of the development in the offshore reservoir in the sparse-well condition.