Combining sedimentary forward modeling with sequential Gauss simulation for fine prediction of tight sandstone reservoir

Abstract

A considerable number of tight sandstone gas fields are observed in the Xihu Depression, East China Sea. The Y gas field is at the stage of development. It is necessary to describe the reservoir shape and permeability characteristics in detail to deploy development wells. The seismic prediction effects of tight sandstone are not good. Sedimentary forward modeling technology can effectively predict the macroscopic characteristics of sedimentary sand bodies, but this technique lacks micro-details. Geostatistical modeling can depict reservoir details, but this technique cannot easily reflect macro-laws. Therefore, taking H1 layer of the Oligocene Huagang Formation in the Y gas field as an example, the combination of sedimentary simulation and geostatistical modeling is proposed. Geostatistical modeling is combined to describe reservoir details by using the sequential Gauss simulation method under the control of sedimentary simulation results as macro-laws. First, based on the test data and the sensitivity test results of the adjacent study area, five parameters (transport capacity of the river, sand ratio, maximum initial water depth, lake level change, and tectonic subsidence) needed for the simulation are determined. A sedimentary forward simulation is carried out, and a sand ratio model reflecting macroscopic sedimentary law is obtained. Second, in the geostatistical modeling stage, the variogram which has a great influence on the results of geological modeling is discussed. The results show that the larger the length and width of the variogram, the larger the simulated reservoir scale. Then, considering the characteristics of fewer wells in the study area, a reliable variogram is obtained by using the sand ratio results of the sedimentary simulation (2538 m in length and 876 m in width) and the main input parameters of geological modeling (data probability distribution characteristics and variogram) are determined. Subsequently, under the trend of the sediment sand ratio simulation, geological modeling research was carried out by using the sequential Gauss simulation method, and a high-precision sand ratio prediction model was established. Finally, the sand ratio model is transformed into a sedimentary microfacies model and the permeability characteristics of different sedimentary microfacies are calculated. Under the control of sedimentary facies, the permeability model is established by the sequential Gauss simulation method. The model prediction results show that the simulation results can reflect the morphology, distribution and permeability of sedimentary microfacies in detail and the macroscopic characteristics of delta sedimentary law. The permeability of channels in the study area is 0.3 md, and the distribution area is much larger than that of other types of sedimentary microfacies. Compared with other types of deposits, the channel around well A area has better development potential in the front of the delta and is the most favorable oil and gas development area in the study area.