A Rock Type Based Evaluation Method for Unconventional Reservoirs

Abstract

The evaluation of fluid saturation in unconventional reservoirs, specifically the mudstone reservoirs, has long presented unique challenges to the petrophysical community. This has led to the industry, for the most part, to use electrically based analysis (such as embodied in the Archie relationship) even though the rocks being analyzed do not fit the characteristics in a classic Archie rock type. In addition these conventional work flows ignore fine scale variations in rock properties that are well documented in unconventional mudstone reservoirs and which are well below log resolution. There are several non-electrical based workflows documented in the literature, but each of these requires specialty log suites not readily available in most mudstone plays. This paper proposes a methodology where unconventional rock types are defined by clay volume, porosity, and water saturation based on core measurements that more accurately represent the storage capacity of these reservoirs. This method recognizes that there is more to the determination of a reservoir’s storage capacity than a simple manipulation of a conventional work flow. While this rock type based approach is very core dependent, the industry has gathered tremendous volumes of core data which for the most part are highly underutilized once an initial reservoir evaluation has taken place. This method does require that we continue gather core data, but it also outlines a methodology that enables the use our data, both new and legacy, in a more rigorous manner. The issue of matrix deliverability is discussed, but until the physics of fluid flow in nano-pore systems is better understood prediction of matrix efficiency will remain an enigma. The use of a rock type based method for unconventional reservoirs allows better predictability for field wide hydrocarbon pore volumes. The petrophysical rock type workflow outlined in this paper can be correlated to rock types are based on depositional and digenetic processes which are better understood and are superior to simple trend maps based on calculated reservoir properties. Adoption of this new approach will lead to improvement of our understanding field wide productivity and allow for the delineation of sweet spots essential for the economic viability of unconventional reservoirs.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018