PETROPHYSICAL DESCRIPTION AND ASSESSMENT OF STORAGE AND FLOW CAPACITY IN THE TRIASSIC KURRACHINE DOLOMITE RESERVOIRS OF ASH SHAER FIELD, PALMYRA, SYRIA

摘要

The use of a multimineral log evaluation model and the availability of cores, borewall image logs, nuclear magnetic resonance logs, dipole sonic logs and well test results helped to reappraise the development strategy for the Ash Shaer gasfield in central Syria. The Triassic Kurrachine Dolomite Formation at the Ash Shaer gasfield, comprises repeated sequences of mudrocks, dolomitised carbonate mudstone and wackestone, peritidal limestones, subaqueous anhydrite and halite. These were deposited in a restricted basin that was intermittently connected to the Neo-Tethys Ocean. Almost all of the observed porosity in the Kurrachine Dolomite is secondary porosity. Early dolomitisation processes occurred at shallow depth by seepage of saline brines associated with evaporite deposition. Burial dolomitisation processes, such as displacive dolomite veining, created euhedral crystal-lined porous and permeable 'zebra structures' and dolomite breccias. Some of these were partly or completely filled by anhydrite cements. Results of fluid inclusion studies suggested that the timing of reservoir hydrocarbons' charge was after the late-stage burial dolomitisation and involves high temperature hydrothermal fluids. The original Kurrachine limestones were deposited in low energy subtidal and peritidal conditions, they were very fine grained and often argillaceous. Only limited primary porosity in the peritidal limestones remains and this appears to be where the fenestral pores were not completely cemented. Natural fractures occur mainly within the limestones but their propagation was limited by less competent dolomite and anhydrite rocks. Natural fractures in these carbonates are consistently aligned and appear to be regional layer bound joint sets rather than faults as no significant displacement is observed. Fracture density may also be related to individual bed thicknesses. These fractures offer very low storage capacity but contribute to gas production rates. Repeated depositional successions are highly correlatable such that reservoir connectivity of individual beds is assured, although porosity and permeability may vary laterally. Late stage dolomitisation in the productive Middle Kurrachine Dolomite correlates at a twenty metre-thick reservoir unit across Ash Shaer and neighbouring fields, suggesting that cubic kilometers of this unit were affected. A detailed study of available core provided an accurate description of the reservoir lithologies and cycles revealed a geometrical relationship between flow capacity (permeability) and storage capacity (dynamically effective porosity plus micro-porosity) for each reservoir facies.