Relationship between Active Deformation, Stress Heterogeneity, and Fracture Permeability in the Suban Gas Field of South Sumatra, Indonesia

Peter Hennings¹, Bambang Pujasmadi², Hugh Alley², Chris Zahm¹, Reid Ray³, Milt Enderlin¹, Bob Lee², and Roland Kirschner^2
1 ConocoPhillips Subsurface Technology, Houston, TX
² ConocoPhillips Indonesia Inc. LTD, Jakarta, Indonesia

Suban Field is located in the actively deforming South Sumatra backarc basin. The field's most productive reservoir bodies are fractured pre-Cenozoic igneous and metamorphic rocks and overlying Cenozoic carbonates. The complex architecture of Suban Field was unraveled using PSTM and PSDM 3D seismic volumes. The structure is a composite of Paleogene extensional elements which were modified by Neogene contraction to produce basement-rooted forced folds along its eastern flank and more shallowly-detached fault-propagation folds along its western flank.

Well test, wellbore image, and drilling data were integrated into a geomechanical framework to develop a stress and fracture characterization of the field. Regional directions of SHmax trend NE but areas of the field show significant stress variation such that SHmax trends NW, paralleling the grain of local structures. We interpret these stress rotations to be result of outer arc extension in active folds and/or the result of co-seismic stress relaxation.

Natural fractures were differentiated into hydraulic classes based on reconciliation of petrophysical character with drilling, well test, and other petrophysical data. Discrete fracture network models were constructed so the fractures could be characterized in their correct spatial framework. Field-wide the spacing of all fractures varies from 0.45 to 1.52 fracs/m. The spacing of fluid-conductive fractures is an order of magnitude less. Undifferentiated, fracture density has no relationship to well productivity while the density of the largest fluid-conductive fractures is strongly correlative. Fluid-conductive fractures occur mainly in fault damage zones and domains of tight folding but their productivity is most clearly controlled by the interaction of stress and local structural elements. The vast majority of fluid-productive fractures occur in an envelope that trends 60° around SHmax.