Porosity model and its application in tight gas sandstone reservoir in the southern part of West Depression, Liaohe Basin, China

The Liaohe basin is a Tertiary rift developed on a paleohigh, characterized by active fault subsidence. Western Depression, one of the most productive areas in Liaohe basin, has been a long-term depositional and subsidence center since Cenozoic. The Shahejie Formation (Es) in the Western Depression are not only the most important source rocks responsible for the major hydrocarbon accumulations, they are also the major reservoir rocks. The third member of Shahejie Formation (Es3) mainly develops sublacustrine fan tight gas sandstones and its sandstones are mainly lithic arkoses, texturally and compositionally immature. The article has developed an origin-based porosity model relying on geologic models and concepts, based on the correlation between sedimentary facies and diagenesis. The porosity prediction model is established on the analysis of the following steps: (1) establish diagenesis index (ID), the function of temperature, Ro, quartz overgrowth, I/S, depth, and time; (2) calculate sedimentary facies index (IF), which is established based on the distribution of porosities in different sedimentary facies during eogenetic stage, and is higher in the sedimentary facies with higher porosity; and (3) build the porosity model based on the relationship between the exponential porosity function of ID and the compaction curve of IF and depth by the linear regression method. This model not only predicts the present-day porosity but also reconstructs the porosity history. The results show that the present-day porosity decreases with increasing diagenetic degree from basin boundary to basin center. And the reservoirs in braided channel and its lateral and front edges of middle fan have higher porosity. The results show that from the early period of deposition to the end of Es32 deposition, Es33 sandstones were in the eogenetic stage with the porosity approximately varied from 35.6% to 14% by mechanical compaction. During the deposition of Es31, sandstones were mainly in the mesogenetic stage Al -A2 undergoing the continuous compaction along with quartz and carbonate cementation. The porosity of Es33 sandstone decreased from 14% to 8%. Moreover, a large amount of secondary porosity was formed with the porosity approximately increased by 2.45%. Up to present-day, Es33 sandstones are mainly in the mesogenetic stage B during the slow subsidence. Most parts of Es33 sandstones are becoming tight with the porosity fell to 4.8% by late compaction and cementation. The improved porosity model not only predict deep reservoir -quality distribution at a more detailed field scale, but also is widely used to other tight sandstone fields that have similar geological background and to identify sweet spots for production. (C) 2016 Elsevier B.V. All rights reserved.