The line between conventional and unconventional petroleum accumulations can become very blurred at times, leading to poor understanding and underutilization of petroleum systems fundamentals. Schmoker (2005) proposed a definition of unconventional systems where “hydrocarbons exist independently of the water column.” As is highlighted in this case-study of the Ordovician sandstone play, understanding the larger petroleum system and its key processes are paramount to improving the exploration and development of a play regardless of its classification as “conventional” or “unconventional.”
In northern Saudi Arabia, the unconventional gas exploration effort has resulted in the discovery of a new, complex, and high impact sandstone play. The play is confined to glacial ribbon channels defined by diagenetically altered sandstone reservoirs. The reservoir facies contains “conventional” porosity and permeability, but the depositional heterogeneity, fluid distribution, and hydrocarbon production from “tight gas” lend to discussion about the nature of the accumulations. Key petroleum system elements dictating the current state of hydrocarbon accumulations were defined. The regional geologic history and environment of deposition were controlled by glaciation, major structural elements, sediment supply, and periods of uplift and erosion. The system is further complicated by the overprint of igneous activity. These variables control the distribution of the potential sources, reservoir, seal, and trapping mechanism. While two potential source rocks exist, the overlying shale is historically credited with generating the gas discovered as well as providing a regional seal.
Extensive work on the sedimentology of the channel sandstones has resulted in a comprehensive understanding of factors influencing reservoir quality. Predictability of reservoir distribution has improved significantly through integration of geologic and geophysical data with fluid distribution providing the next obstacle to address. The depositional and diagenetic controls on reservoir quality have been identified, but their lateral and vertical variability still creates a high level of reservoir compartmentalization. Variations in porosity and permeability, with the lithologic changes at the natural channel boundaries, form stratigraphic trapping mechanisms. The structural history and associated faults introduce an added layer of complexity that requires further study as they have potential to act as both seals and leaks. Pressure data shows variable gradients in both water and gas systems, yet, overall, there is evidence of a larger and active hydrodynamic system.
The initial factors that lead to ambiguity in the classification of conventional vs. unconventional warrant additional investigation into utilization of unconventional practices, including technology, workflows, and team structures. The channel play in northern Saudi Arabia highlights how quantification of petroleum system fundamentals are as critical as using unconventional practices and approaches for an exploration and development program to be as successful and efficient as possible.