Heterogeneity in glaciogenic reservoirs: examples from the Ordovician and Permo-Carboniferous of Saudi Arabia

May 25, 2010

Guest Speaker: Dr. John Melvin,
Geological Consultant, Saudi Aramco

Abstract:
Heterogeneities in glaciogenic reservoirs abound, and give rise to a complex reservoir architecture which derives from a disparate depositional facies mosaic that may be significantly further complicated (within synglacial deposits) by the effects of glacial tectonics, as well as numerous extremely irregular bounding surfaces related to multiple phases of glacial advance and retreat. These heterogeneities give rise to real difficulties in predicting the occurrence of reservoir quality facies (at an exploration scale), and may create problems of reservoir compartmentalization during subsequent field development. 
During the Ordovician (Hirnantian) glaciation, the initial glacial advance was manifest in widespread, deformed and extremely poorly sorted, boulder-bearing diamictites of the Zarqa Formation. These are non-reservoir rocks that sit upon a major unconformity. They are overlain by sandstones and conglomerates of the Sarah Formation, which commonly occur in elongate paleovalleys that show evidence of having been glaciotectonically enhanced. Significant differences between facies occurring within these paleovalleys compared to their “interfluve” areas give rise to heterogeneity at the largest scale. “In-valley” deposits consist of a variety of depositional facies that occur upon an irregular depositional surface, and comprise a variety of thin-, medium- and thick-bedded “turbidite” sandstones of moderate to good reservoir quality, as well as poor-quality diamictite facies. These facies are found in mutual juxtaposition within the paleovalleys, separated by glacially thrusted material that is ascribed to minor glacial re-advance during an overall glacial retreat phase. Evidence also exists within some paleovalleys of major glacial re-advance, manifest in deformation of the earlier glacial retreat phase deposits. “Interfluve” deposits appear to be of poorer quality, and are related to retreat-phase diamictite facies. The terminal retreat phase of the Hirnantian glaciation is seen in upward-coarsening depositional packages that display shallow marine affinities and are of only limited distribution. Some degree of isostatic rebound can be postulated in places to be associated with these terminal glacial facies.
The Permo-Carboniferous lower Unayzah is associated with the Late Paleozoic Gondwanan glaciation. It also sits upon a major unconformity but contains no evidence of any initial, major glacial-advance diamictite facies (compared with the Ordovician Zarqa Formation). Its lowermost member (Unayzah C) comprises only limited depositional facies thought to have been laid down during phases of glacial retreat upon extensive glacio-fluvial outwash braidplains. The original depositional character of these deposits has been severely compromised by multiple, subsequent phases of glacial re-advance, with associated glacio-tectonic deformation. Thus in its present form the Unayzah C member is characterized by stacked units, many tens of feet thick, of commonly massive sandstones separated by distinctive shear zones that give rise to a significant element of compartmentalization within these reservoirs. The terminal retreat phase of this late Paleozoic glaciation in Saudi Arabia (the Unayzah B member) is characterized by an extremely diverse suite of basin-central glacio-lacustrine facies (including massive diamictites, stratified diamictites, sub-lacustrine gravity flows, and mudrocks with dropstones) with braided fluvial sands and gravels around the basin margins. These deposits all show evidence of continued and extensive glacial melting, resulting ultimately in the formation of a severely flooded late-glacial landscape dominated by extensive lakes. The end of the glaciation is marked by a very abrupt change in facies, interpreted as a massive drainage event.
Heterogeneity thus appears to be characteristic of glaciogenic deposits, whatever their age and location. This intrinsic variability occurs from a regional scale to field-wide reservoir scale. It is the product not only of a wide range of syn- and late-glacial depositional facies, but also of the effects of widespread synglacial deformation. The likelihood of such heterogeneity will significantly impact predictability of occurrence of attractive reservoir facies (in exploration), as well as the likelihood of difficulties of reservoir compartmentalization in reservoir development.

Biography:
John Melvin is a Geological Consultant in Saudi Aramco’s Reservoir Characterization Department (RCD), where he recently assumed responsibility for a major new Mentorship Program aimed at developing expertise in siliciclastic sedimentology and stratigraphy among young Saudi geoscientists. Prior to adopting this position, he was team leader for the Gas Fields Special Studies Team within RCD’s Gas Fields Characterization Division. There, he characterized the clastic sedimentology and reservoir stratigraphy of Carboniferous to Permian Unayzah and Basal Khuff Clastics reservoirs, as well as Devonian Jauf reservoirs, while in the process completing the description and interpretation of over 22,000 feet of core. His work provided critical input to reservoir geocellular modeling and simulation efforts for almost all of Saudi Aramco’s Paleozoic reservoirs, while at the same time significantly rationalizing the subsurface stratigraphy of the Unayzah. John’s earlier projects with Aramco involved sequence stratigraphy of the Upper Ordovician Sarah Formation and of the Silurian Qusaiba Member.
After obtaining his Ph.D. from the University of Edinburgh in Scotland, John spent over twenty years as an applied sedimentologist with BP, involved in a large number of exploration and development reservoir studies in the North Sea and Alaska. He then spent six years as a consulting sedimentologist and stratigrapher, successfully concluding reservoir studies in Egypt, Libya, Colombia, and the North Sea, before joining Saudi Aramco in 2001.
John has published several articles on applied sedimentology and stratigraphy. He is a member of the American Association of Petroleum Geologists (AAPG), the Dhahran Geoscience Society, the International Association of Sedimentologists (IAS), and the Petroleum Exploration Society of Great Britain (PESGB).