Effects of Deep Burial Late Corrosion on the Porosity Distribution in Isolated Tertiary Build Up Reservoirs; An Example from The Luconia Province, Malaysia

G. Warrlich¹, M. Esteban²,C. Taberner³, A. Dombrowski⁴, J. H. Van Konijnenburg⁵, M. Rejas³
1. Shell International E&P, Rijswijk, The Netherlands, [email protected]
2. Carbonates International Iberia. Mallorca, Spain, carbonates@carbonates-org
3. Institute of Earth Sciences, CSIC. Barcelona. Spain, [email protected]
4. Shell International E&P, Houston, U.S.A., [email protected]
5. Sarawak Shell Berhad, Sarawak, Malaysia, [email protected]

Porosity distributions in many isolated Tertiary build-ups are generally attributed to depositional and early diagenetic processes. Therefore a direct link between depositional facies and reservoir rock types is often assumed in the static model building workflow. This can be unrealistic as late corroding and dolomitising diagenetic fluids may overprint porosity distributions originally formed by depositional and early diagenetic processes. This late corrosion at deep burial stages is favoured by fluid circulation along fractures and veins, rock-discontinuities (e.g. bedding planes, re-open pressure-dissolution horizons, and preserved primary or early diagenetic porosity). In this case, the distribution of reservoir rock types becomes more complex and the direct link between depositional facies and reservoir rock types has to be revised. A workflow including fault and fracture characterisation as well as diagenetic studies on core samples and geochemical analysis is necessary to predict the porosity and permeability distribution.

Megabank and Pinnacle units of Miocene carbonates from Luconia (Malaysia) were “revisited” to evaluate the role of deep burial diagenesis vs. currently admitted interpretations of an early diagenetic origin of the reservoir. Mud-rich packstones, wackestones and clay-lean mudstones were extensively dolomitised and selectively corroded by fluids of deep-burial origin. Grainstones and mud-lean packstones with former aragonitic constituents tend to have the worst reservoir properties. Skeletal moldic porosity is a diagenetically late development in originally calcitic constituents. Overdolomitization and late calcite cementation locally affected the general, layered reservoir pattern. The available data and interpretations do not support early diagenetic (shallow burial) origin of the dolomite, nor the preservation of early diagenetic porosity into present-day reservoir conditions.

Fluids in equilibrium with dolomite and late calcite, at the homogenization temperatures (90 and 110ºC) recorded in their primary fluid inclusions, had a similar value (¹⁸O ≈ +2 to +6 ‰SMOW). This indicates that both dolomite and calcite precipitated from a same fluid. Interpreted ¹⁸O values of parental fluids agree with those recorded in formation waters present in some wells (¹⁸O = -0.3 and +5.0 ‰ SMOW).

Available data are consistent with a model of mixing fluids during burial to explain the late dolomitisation, several pulses or waves of corrosion and late cementation. These conclusions have strong implications at the stage of static model building workflow for reservoir evaluation.

The authors would like to thank PETRONAS for permission to use the data and publish the results in this presentation.