AAPG Geoscience Technology Workshop

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The disruptive knowledge on key controlling factors for deeply buried carbonate reservoirs

Abstract

Carbonate E&P in Asia has led to new and challenging facts to carbonate geology. In Tarim Basin west China, large Paleo-caves and extensive cave systems are still “alive” (void without collapsed) at depth down to 7000m in Ordovician carbonates. They are evident in seismic as strong “pair of beads” bright-spots and proved in drilling with large mud-losses and bit-drop and in production with abnormal high flowrate and early water breakthrough. Similar cave features and networks have been recognized in Miocene reef build-ups in the Central Luconia Basin, offshore Malaysia. which has raised a critical question to current knowledge on carbonate reservoir: how extensive cave systems in dendric pattern could be formed in isolate reef build-ups at middle of ocean without evident fault and unconformity. An integrated study has been performed with a data-driven approach on worldwide carbonates. This effort has led to a new insight into carbonate rocks supported with seamless evidences from deposition to production. Large cavities in carbonate, such as vugs, cave, and cave systems, have originated primarily during carbonate building up with coral differential growth and tide-channel development. As a result, the original heterogeneity in carbonate could be extensive and extreme with large cavities up to kilometre long distributed in relatively tight matrix. This primary heterogeneity has significant impact on concurrent and subsequent diagenesis and dissolution. The diagenesis has impacts mainly on small matrix porosities, reducing their sizes and connectivity gradually from porous to tight carbonates. The dissolution or karstification has impacts mainly on large cavities, basically increase their sizes and connectivity. This new insight on carbonate heterogeneity is a disruptive breakthrough in carbonate geology. It reveals that for reservoir quality in deeply or ultra-deeply buried carbonates the primary key controlling factor is the large cavities of primary origination. The diagenesis has limited impact on large primary cavities. The dissolution/karstification could not generate large and extensive cavities in tight matrix. This understanding scientifically explains the deep reservoir quality in Tarim Basin. In three areas of Tarim, cavity connectivity is quite different even they are in the same basin and with quit same characters in seismic. These differences lead to different main risk and best practices in development operations. • In Lungu area at east of north basin slope, carbonate deposited in land-attached environment with hills, rivers and valleys widely developed at high relief carbonate top. Cavities are well connected both vertically and laterally forming extensive networks. Flow-units are large with water-breakthrough as main risk sediment-fill as secondary risk. • In Halahatang area at west of north basin slope, carbonate deposited in land-attached environment with meandering rivers developed at moderate relief carbonate tops. Cavity connection is good in vertical but poor in lateral. Flow-units are large with water-break as through main risk. • In Tazhong area at central uplift of basin, carbonate deposited in land-detached platform environment without any river features identified at flat carbonate top. Cavities are isolated with poor connectivity in both vertical and lateral. Flow-units are small with sediment-filling as main risk and water-breakthrough as secondary risk.