Abstract: Developing an Integrated Exploration Strategy for Complex Carbonate Reservoirs in a Tertiary Wrench Segmented Basin: Bohai Bay, China
NEESE, DOUGLAS G., and GREGORY A. JEWELL, Kerr McGee Corporation, Houston, TX
Variations in contrasting carbonate depositional/diagenetic settings (Oligocene lacustrine grainstone shoals/Ordovician karst) provide attractive hydrocarbon exploration targets, on paleo-highs in the structurally segmented Tertiary Bohai basin. Oligocene “Shahejie” carbonates lie disconformably upon Ordovician karsted and fractured dolomitic limestones. Initial production tests in two wells range from 4-6 MBOPD in the carbonate reservoirs. Strain accommodation by Tertiary transtensional tectonics has resulted in severe structural overprinting of Paleozoic faults. Seismic resolution of original carbonate reservoir geometries, paleo-topography, and present day structure is hampered by 1)episodic tectonics during the Tertiary, and 2)layered Miocene basalts reducing the signal to noise ratios needed for seismic imaging of deeper buried hill carbonate reservoir complexes.
The transition from Ordovician karst to the overlying 20-30m thick Oligocene Shahejie Fm. carbonate unit, cannot be imaged by 2-D seismic. Carbonate depositional sequences and their corresponding sequential reservoir rock units were subdivided by textural and compositional variations, determined through whole core analyses and FMI image logs. The middle to lower Ordovician section is comprised of predominantly marine non-porous dolomitic limestones containing variable amounts of cemented and open fractures. Overlying this are several brecciated paleokarst fabrics, imaged by FMI from the well-bore. Mosaic and crackle breccias are present in the upper 25 meters of Ordovician carbonate with porosities ranging up to 15%. A regional distribution of paleo-karst has provided a Bohai Bay buried hill substrate for subsequent variable morphologies of reservoir quality Tertiary lacustrine carbonates. These carbonate facies include: 1)interbedded gastropod-rich grainstones to packstones, 2)mixed skeletal/non-skeletal wackestones to algal mudstones, 3)oncolitic rudstones to floatstones, and 4)intraclastic breccia debris. Grain dominated fabrics contain vuggy, interparticle, pinpoint, and moldic porosity types of up to 24%. The best porosity occurs in the multi-phase diagenetically altered, gastropod rich zones.
Subsidence analysis shows the critical point of the petroleum system to be the onset of hydrocarbon generation occurring at 4-5 Ma coincident with high sediment accumulation rates during the Plio-Pleistocene, with continued generation and accelerated transformation of kerogen to the present day. The seismically mapped structural crests of the productive carbonate reservoir lie within the presently generating hydrocarbon window, based on kinetic modeling utilizing available vitrinite/temperature data. By utilizing an integrated exploration approach, the complexities of the Bohai basin can be better resolved.