The Influence of Facies and Fracturing on the Petrophysical Properties of Carbonates
Joyce Neilson¹,², David Healy¹, Ian Alsop¹, Thomas Haines¹, Emma Michie¹, Nicholas Timms³, and Moyra Wilson³
¹University of Aberdeen, UK
²Carbonate Reservoirs Ltd., Aberdeen, UK
³Curtin University of Technology, Perth, Australia
The reservoir properties (especially permeability) of carbonates are notoriously difficult to predict. This arises from the well known and documented variability in pore systems in carbonates, partly the result of initial facies but also due to extreme variability in the effects of diagenesis (cementation and dissolution) and structural evolution (faulting and fracturing).
In order to investigate the effect that rock texture and fracturing has on the petrophysical properties of carbonates, an outcrop study has been designed to record facies, degree of diagenesis, style and amount of faulting and fracturing and sampling to obtain porosity, permeability, Vp and Vs measurements in the laboratory. Oligo-Miocene carbonates cropping out on the island of Malta in the Mediterranean, were chosen because of their geological simplicity and limited degree of diagenesis. In this way it is hoped that the effects of initial facies on the pore systems and the faulting and fracture style and intensity can be identified and measured at outcrop and, using Effective Medium Theory, predicted in the subsurface.
During the Oligo-Early Miocene, major rifting and re-organisation occurred in the Western Mediterranean and to the east, the Gulf of Aden-Red Sea- Gulf of Suez rift system developed. The carbonates that now comprise the Oligo-Miocene of the the present-day Maltese Archipelago however were deposited offshore Africa in a relatively quiet area of the Mediterranean. Only minor evidence of tectonic movement is observed, characterised by minor fault-controlled thickness changes and the development of neptunian dykes (Meulenkamp and Sissingh, 2003). By the Pliocene however, the Tyrrhenian Basin had opened up to the north of the Maltese Archipelago accompanied by rifting in the Pantellaria Rift just to the south with uplift of the northern rift flank (i.e., the present-day Maltese Archipelago). It is this movement that is the origin of the major faulting observed in the islands today (Hill and Hayward, 1988).
The Oligo-Miocene sediments in Malta developed in an outer-ramp setting around the northern margins of the Southern Peri-Tethys Platform (northern margin of the African plate) on a relatively stable platform. They comprise the Lower Coralline Limestone composed of grainstones and bioclastic packstones and the Globigerina Limestone which is divided into the Lower (bioclastic wackestones/packstones), Middle (micrictic) and Upper (micritic) Members. Each of these is separated by a hardground with several intervening firmgrounds. The limestones are capped by the Blue Clay, a Mid Miocene deep water clastic unit deposited in up to 150m water. As such, a variety of initial facies types have been studied across the island. The nomenclature follows that of Pedley et al. (1976).
AAPG Search and Discovery Article #120034©2012 AAPG Hedberg Conference Fundamental Controls on Flow in Carbonates, Saint-Cyr Sur Mer, Provence, France, July 8-13, 2012