Kela-2 Field: Structural Model and Risk of Fault Compartmentalization

Fred Dula, Eddie McAllister, Amgad I. Younes, Jonathan Pugh, and Akke-Marijke Gunst
Shell International Exploration and Production, B.V, Rijswijk, Netherlands

Kela-2 is a major overpressured gas accumulation in the northern Tarim Basin, China. A structural model was developed to: 1) explain the geometry of the top reservoir, 2) constrain the southwest flank of the anticline where steep dips and poor seismic reduce mapping confidence, 3) clarify the structural evolution, and 4) assess the likelihood of compartmentalization due to intra-field faults.

The structure consists of three south-verging thrust faults with one major and one minor north-verging back-thrusts. The south-verging thrusts form an imbricate stack which collectively comprise a wedge that indents the Tertiary overburden of the Baicheng Depression. Each thrust sheet is bounded on the top and bottom by a thrust fault, with a constant imbricate thicknesses being maintained across the entire structure. Horizon ages, restorations, and basin modelling constraints indicate the Kela-2 imbricate stack has developed within the past 5 m.y.

Intra-reservoir faults were analyzed to evaluate compartmentalization. Fault densities, styles and distributions of seismic amplitudes were used to subdivide the field into four structural domains. Reservoir and fault rock properties were modelled to predict the dynamic behavior of faults. Overpressure development may have prevented normal compaction and cataclasis in fault rocks and helped preserve the relatively high porosities, despite burial depths in excess of 6 km. Modelled cataclastic fault rock permeabilities range from 1- 50 mD and are facies dependent. Given these fault rock permeabilities and the associated average matrix permeabilities, intra-reservoir faults are predicted not to significantly baffle gas flow during production.