225 / 2019-06-28 09:13:53

Ichnofabrics and their role in the modification of petrophysical properties: a case study of the Ordovician Majiagou Formation, Northwest Henan Province, China

Ichnofabric; Bioturbation; Carbonate Reservoir Analogue; Sedimentary Environment; Ordovician

The carbonate rocks of the Ordovician Majiagou Formation from the northwestern part of Henan Province and their diverse trace fossils provide an outcrop analogue for bioturbated, low porosity and low permeability carbonate reservoirs. Three ichnofabrics can be defined according to their characteristic trace fossils and their ichnodiversities, individual abundances and overall bioturbation indices. The Balanoglossites ichnofabric indicates relatively high-energy sedimentary environments such as intertidal zone and subtidal shoals in the carbonate platform. The Planolites ichnofabric suggests relatively low-energy, protected sedimentary environments such as lagoons in a restricted carbonate platform but also occurs in sediments deposited in deeper-water depressions in the open carbonate platform. The Chondrites ichnofabric characterizes low-energy and reduced oxygen conditions, with burrows penetrating below a shallow redox interface in deeper-water settings that include the distal slope as well as local depressions in the open carbonate platform.
Our analysis suggests that the modifications of petrophysical parameters are controlled or influenced by the ichnofabric characteristics (burrow architecture, burrow incision relationship, bioturbation index and burrow connectivity), the permeability contrast between burrow-fills and matrix, and the dolomitization of burrow-fills. The micritic host sediments have negligible porosity and permeability. The bioturbated sediments have significant differences in fabric, texture, porosity and permeability. The burrow-fills are very commonly dolomitised. Each of the burrow-types can enhance reservoir properties, but the greatest differences are associated with the Balanoglossites ichnofabric, in which porosity and permeability can reach over 2% and 70 mD. Preliminary stratigraphic studies indicate that reservoir potential in the subsurface will be linked to an understanding of the distribution of shoal and intertidal facies as well as their associated Balanoglossites ichnofabrics.