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GCActive Gas Chimneys and Oilfield Karst Associated With a Miocene Reef Complex: Liuhua 11-1 Field, South China Sea*
By
Chip Story1
Search and Discovery Article #20013 (2003)
*Adapted for online presentation from the Geophysical Corner column in AAPG Explorer, June, 2002, entitled “3-D Images Active Gas Changes,” prepared by the author, and based on presentation, with co-authors Christoph Heubeck, Free University of Berlin, Germany; Patrick Peng and Claire Sullivan, BP; and Jian Dong Lin, China National Offshore Oil Corpation, at 2002 AAPG annual meeting in Houston. .Appreciation is expressed to the author and to R. Randy Ray, Chairman of the AAPG Geophysical Integration Committee, and to Larry Nation, AAPG Communications Director, for their support of this online version.
1Vision Resources, Houston, Texas
The Liuhua 11-1 Field, located 130 miles southeast of Hong Kong under 1,000 feet of water in the Pearl River Mouth Basin (Figure 1), was discovered in 1987 and is currently being developed by the consortium of BP, China National Offshore Oil, and Kerr-McGee. The reservoir zone at 3,850 feet subsea is producing 16-22 API degree oil through 25 long-radius horizontal wells.
The
Liuhua Field is bounded by high-water-flow faults and karst features that affect
the production of bottom water within the heavy oil reservoir. Three-D seismic
data reveal details of the reservoir heterogeneity in spectacular images of gas
chimneys associated with both linear and circular karst features. An ultra
high-resolution 3-D
seismic
survey over Liuhua was acquired in July, 1997. With
peak frequencies over 200Hz, the
seismic
data have allowed for temporal and
spatial resolution on the order of 14 feet. Faults, fractures, and karst
features in the reservoir were analyzed on this dataset using coherence
technology.
Complex attribute
analyses added a greater understanding of rock matrix
continuity, which was initially thought to provide a tight, competent seal to
underlying aquifers. The focus of this article is on carbonate solution collapse
and the associated development of gas chimneys
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The Liuhua reef carbonates are projected to
have in-place reserves of 1.2 billion barrels. After the initial
production in 1996 peaked at 65,000 BOPD but declined rapidly, it became
clear that the reservoir lithology was more petrophysically
heterogeneous than originally thought and that a 3-D A structure map of the top of the reef (Figure 2) shows bounding faults on the north and south sides of the Liuhua reservoir. The southern fault system is associated with several circular karst-collapse structures clustered south of the production platform. Figure 3 is an enlargement of this area from a coherence image showing the internal detail of these features and a modern analog in Belize.
Oilfield Karst and Gas Chimneys The gas chimneys associated with karst leaching are caused by the CO2, H2S, and methane byproducts of the bacterial degradation of the oil. The actual karst-collapse results from carbonic acid dissolution associated with the generation of the CO2.
In the Liuhua reservoir the major faults
provide channels for significant vertical movement of water at the edges
of the reservoir. Several poor quality wells have been drilled into or
near these fault zones. At the same time, the ongoing karst solution
collapse, which appears to have been active for almost 15 million years,
also creates vertical zones for water encroachment both outside of and
within the productive area of the reservoir. Figure 4 is a Geochemical and mechanical effects caused by dissolution microfracturing and stratigraphic brecciation of the brittle carbonate matrix ultimately create pathways for the upward movement of water into the horizontal well bores. Tight rock appears to become more permeable, while porous reservoir rock becomes less porous and permeable as a result of these combined processes. Because of the preferential permeability of water relative to oil in a heavy-oil reservoir, the tighter rock now produces water almost to the exclusion of oil. Figure 5 is a north-south section showing the vertical dim amplitude zones, gas sag, and collapse adjacent to the bounding faults. Both groups of bounding faults are adjacent to partially collapsed gas time-sag zones within the reservoir. This subtle low velocity sag (about four meters) is linear rather than circular and is thought to represent incipient carbonate dissolution. Figure 6 is an east-west reflection strength section within the chimney zone, parallel to the south edge of the reservoir in Figure 5. Again, the amplitude anomaly that extends to the sea floor in the chimney collapse zones within and above the reservoir is due to gas, suspected microfracturing, and some carbonate porosity changes. This same zone is connected to the large off-structure sinkhole complex shown in Figures 2 and 3 and was modeled as a major source of water influx responsible for poor production in the western field area. Much of the prior geoscience understanding of the Liuhua reservoir was revised as a result of this work, including:
The field fluid movement was modeled successfully in a reservoir
simulation guided by |