Dolomitization and Reservoir Development of Shallow-Water Carbonates: Lower Triassic Feixianguan Grainstones in Eastern Sichuan Basin (SW China)
The lower Triassic Feixianguan grainstones are significant gas reservoirs in Eastern Sichuan basin. A typical feature of these deeply buried reservoirs is the strong relation between porosity and lithology: dolostones are commonly porous and they can exibit porosity in excess of 20% at a depth of more than 6000 meters, whereas the equivalent limestones and dolomitic limestones are generally of porosity below 2%.
Dolostones are preferentially distributed along platform margins; and dolomitization was prone to occur in middle to upper part within a shallowing-upward cycle. Petrographical observation resulted in identifying (i) two types of dolostones, i.e. fabric-retentive and fabric-destructive ones; (ii) one phase of replacive dolomite (microcrystalline to finely crystalline, nonplanar dolomite replacing ooids or other grains) and (iii) two phases of void-filling dolomite cements (medium to coarsely crystalline, planar-e dolomite & coarsely crystalline, planar-s or planar-e saddle dolomite). Replacive dolomitization is interpreted to have been occurred penecontemporaneously by evaporative seawater that refluxed basinward, based on the following evidences: (1) dolomitization occured in sediments that are relatively uncompacted, and the degree of preservation of original fabrics is quite significant in microcrystalline and finely crystalline dolostones, (2) dolomites are mostly fine (<30µm-120µm), non-luminescent to very dully luminescent, (3) a large portion of dolomites are non-stochiometric, (4) carbon and strontium isotopic values can be comparable to Feixianguan seawater signatures, and(5)vertical distribution of alternating dolomitized and undolomitized intervals indicates periodic dolomitization. However, early-formed dolostones have undergone variable degree of recrystallization in burial environment, resulting in fabric-destructive, coarser dolostones with elevated 87Sr/86Sr and depleted 18O.
A volume-per-volume replacement model prevailed in the dolomitizing process, and intercrystalline porosity is rarely created. Porosity of Feixianguan dolostones is primarily inherited from precursor limestones. Early dolomitization played an important role in reservoir development through retaining much of pre-existing porosity, owing to more resistance of dolostones to compaction and resultant less carbonate cements in the subsurface.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California