Sand connects
In complex non-marine sequences common in the Carboniferous and Permian of the Middle East (and other parts of Gondwana), the connectivity of sandstone units between wells is often difficult to determine particularly where seismic is poor. We might have identified the sands on well logs, but we may not know the extent of sub-seismic mudstone baffles to fluid flow, or if a sand identified in one well is the same as a sand in another well, even in a well close-by. In the absence of pressure data or with low well density we would find it hard to determine the effective capacity of the sandstone units. This applies as much to oil and gas as it does to other uses that we might put subsurface sandstone reservoirs to, for example groundwater extraction, carbon capture and storage, or aquifer heat/coolth extraction or storage.
To solve a problem like this, there is no substitute for studying the rocks in the field – where you can see how rock facies change through successions vertically and laterally, and where 3D visualisation is possible simply by looking. Combined fieldwork and palynology of this type in Jordan and Oman in non-marine Permian alluvial flood plain successions of the Umm Irna and Gharif formations has shown how reconstruction of paleoenvironments could help to understand sand connectivity and the lateral persistence of baffles to fluid flow in subsurface sandstone reservoirs.
First Jordan. Work in the alluvial plain sediments of the Upper Permian Umm Irna Formation (Stephenson and Powell 2013, 2014), which is similar to the Gharif Formation in Oman, parts of the Unayzah Formation in the Gulf, and the Warchha and Sardhai formations of Pakistan allowed the reconstruction of a detailed paleoenvironmental model (below).
Paleoenvironmental-sedimentological model of the Umm Irna Formation, Jordan (from Stephenson and Powell 2013)
Palynological study of numerous argillaceous units preserved in this floodplain complex, outcropping for 40km north to south along the Jordanian Dead Sea coast, allowed an understanding of the formation of baffles in the sand dominated succession. The argillaceous units revealed assemblages of fossil pollen and spores that varied a lot but generally fell into two categories. The first assemblage was found in laterally persistent argillaceous units like migrating point parts or flood deposits associated with main river channels. This was high in diversity, containing a wide variety of pollen and spores that probably represent a regional snapshot of the vegetation on the floodplain and the higher ground around. The second assemblage from smaller argillaceous units like oxbow plugs was of lower diversity with high proportions of one or two local palynomorphs, and also the spores of green algae (zygospores) that usually indicate water bodies that are drying up. Purely on palynological character therefore it’s possible to distinguish a ‘formation wide’ baffle from a less persistent mudstone unit in the Umm Irna Formation.
Palynological study in the pre-Khuff clastics of Oman, tells a similar story. Laterally impersistent argillaceous units characteristic of the these non-marine alluvial sediments, outcropping in the Huqf area, indicate a similar low diversity assemblage which contains numerous zygospores including two new species Schizosporis? pennyi and Tetraporina forbesii which were described in 2011 (Stephenson 2011), and many specimens of the likely zygospore Quadrisporites horridus.
Laterally impersistent argillaceous units in the pre-Khuff clastics of Oman that contain low diversity palynological assemblages and common zygospores. Foreground outcrop about 3m high.
These Jordanian and Oman palynological studies are small but could be used alongside the building of paleoenvironmental and sedimentological models that can help to understand the ‘plumbing’ of complex non-marine successions that are common in the Middle Eastern and Gondwana Carboniferous and Permian. To test if the palynology works more widely would require sampling in other successions with similar palaeoenvironments, and then application of the method in cored sequences of similar facies where pressure data or seismic might validate findings from palynology.
Palynological methods like these won’t solve all the problems on their own but could contribute to the methods at the exploration geologist’s disposal. Developing methods like these may also help with similar successions such as those of the Triassic Sherwood Sandstone Group in the UK, and co-eval formations in Europe, which are the targets for geothermal exploration and carbon capture and storage (e.g. Thompson et al. 2019). Both of these low-carbon energy techniques rely on an understanding of fluid flow for injectivity and extraction.
References
Stephenson, M H. 2011. Two new non-haptotypic palynomorph taxa from the Middle Permian Upper Gharif Member, Oman. Rivista Italiana di Paleontologia e Stratigrafia, 117, 211-219. Open access
Stephenson, M H, and Powell, J H. 2013. Palynology and alluvial architecture in the Permian Umm Irna Formation, Dead Sea, Jordan. GeoArabia, 18, 3, 17-60. Open access
Stephenson, M H, and Powell, J H. 2014. Selected spores and pollen from the Permian Umm Irna Formation, Jordan, and their stratigraphic utility in the Middle East and North Africa. Rivista Italiana di Paleontologia e Stratigrafia, 120, 145-156. Open access
Thompson, J., Parkes, D., Hough, E. and Wakefield O. 2019. Using core and outcrop analogues to predict flow pathways in the subsurface: examples from the Triassic sandstones of north Cheshire, UK. Adv. Geosci., 49, 121–127,
Prof Mike Stephenson is Director of Stephenson Geoscience Consulting