Formation evaluation of a clastic gas reservoir: presentation of a solution to a fundamentally difficult problem

Formation evaluation of clastic reservoirs has always had some difficulties. This issue for a clastic gas reservoir from North-East Iran has already made some problems for petrophysicists. Therefore, this paper presents a basic and consequential strategy for (1) solving the problem regarding the petrophysical evaluation, (2) evaluating the petrophysical parameters as precisely as possible, and (3) presenting a general framework for formation evaluation of clastic reservoirs with complex mineralogy. The applied strategy had three main steps. At the first step, using some conventional methods like petrographical analysis and some petrophysical cross plots, all the minerals in the rock structure were identified as precisely as possible. At the second step, having sufficient logs for a key well, all the identified minerals were modeled to distinguish the main minerals. At the final step, all the main minerals were then used for modeling of the other wells. The evaluation of the formation was basically improved using the applied procedure. Alongside the clay minerals, the results showed an important contribution of other high gamma ray minerals in the mineralogy, such as the mica and feldspars. The results also showed important effects of the precise mineralogy on some petrophysical parameters, such as the porosity, grain density, and water saturation.