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Prediction of Syn-Sedimentary Fractures within Carbonates: Geomechanical Models with Development of More-Realistic Constitutive Models for Carbonates

V. Heesakkers¹, J. Nevitt², W. Narr³, and P. Lovely¹
¹Chevron ETC, Houston, TX, USA
²Stanford University, Stanford, CA, USA
³Chevron ETC, San Ramon, CA, USA

Fractures within steep rimmed carbonate platform reservoirs can have significant impact on hydrocarbon migration, storage and permeability of the reservoir. This paper focuses on the syn-sedimentary fractures which form contemporaneous with the development of the platform. We present 2D geomechanical models to investigate the effect of compaction-induced differential subsidence on the development of syn-sedimentary fractures, and attempt to predict their occurrence and distributions. A significant effort of the mechanical modeling is the development of proper constitutive laws for different carbonate facies of steep rimmed carbonate platforms as their rock types strengthen through time and burial. We calibrate the outcome of our models with field observations from the Devonian carbonate reef of the Canning Basin, W. Australia, and the Permian Capitan reef of the Guadalupe Mountains, NM, USA, both of which are field analogues for major carbonate platform reservoirs like Tengiz and Karachaganak in Kazakhstan.

Prograding carbonate platforms often develop high relief margins with moderate to steep slopes. The carbonate sediments involved in the buildup of such systems consist of distinct platform-, reef- and slope facies. Experimental studies have shown that carbonate sediments undergo significant compaction during the early stages of burial (Goldhammer, 1997) and each of these facies is affected differently by compaction. Platform facies consists dominantly of moderately compactable mudstones and peloidal packstones, where the reef often consists of less compactable grainstones and non-compactable microbial boundstones that lithify at surface conditions. The difference in material properties for each of these facies is the cause for differential compaction during the buildup of the platform. The rapid progradation, leading to over-steepening of the slope, in conjunction with differential compaction of underlying sediments is suggested to be the cause for modification of depositional geometries and formation of faults and fractures (Frost and Kerans, 2009; Hunt et al., 2002; Resor and Flodin, 2009; Stanton and Pray, 2004). The early-formed fractures can strongly impact fluid flow within major carbonate reservoirs like Tengiz and Karachaganak, and are well exposed in outcrop analogues.

The mechanical models of step-wise carbonate platform development, with proper constitutive models adjusted to the strength of different facies, successfully predict syn-sedimentary fracture development and enhance the understanding of the development of these fracture systems. Such models can be used as predictive tools for fracture development in major carbonate platform reservoirs like Tengiz and Karachaganak.

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