^GCActive Gas Chimneys and Oilfield Karst Associated With a Miocene Reef Complex: Liuhua 11-1 Field, South China Sea*

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

General Statement

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

uGeneral statement

uFigure captions

uLiuhua geology

uOilfield karst & gas chimney

uOilfield karst & gas chimney

uOilfield karst & gas chimney

uOilfield karst & gas chimney

uConclusions

Figure Captions

Figure 1. Location map of Liuhua 11-1 Field, South China Sea.

Figure 2. Depth structure map showing central platform and directional wellbores. The map area is 14 X 7 kilometers. Major karst-collapse features shown.

Figure 3. A close-up coherence image of the karst sinkholes on the field’s southern margin. A modern karst analog is the Great Blue Hole, offshore Belize, shown in an oblique areal view and in a close-up.

Figure 4. Reflection strength seismic section over chimney at south edge of reservoir. Structural deformation is highly constrained in a cylindrical pipe. Shale section above reservoir must be microfractured to allow for upward gas/water movement and dim-out within the chimney.

Figure 5. A north-south seismic section illustrating the fault-bounded reef platform and associated gas chimneys rising from the oil reservoir.

Figure 6. An east-west seismic section within the southern chimney zone of Figure 5. Note the amplitude loss in the reservoir carbonates near the base of the chimney and the brightening of some of the more porous shallow units adjacent to the chimney and downthrown to the fault.

Liuhua Geology

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 seismic dataset was needed for a reservoir characterization.

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 seismic reflection strength section showing the chaotic reflectivity associated with the vertical deformation and gas chimney over the large collapse feature at the reservoir’s southern edge. This feature spans about 5,000 feet of vertical section and is rooted at the base of the carbonate platform in a sandstone aquifer that crops out on the seafloor.

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.

Conclusions

Much of the prior geoscience understanding of the Liuhua reservoir was revised as a result of this work, including:

Oilfield karst is now thought to be a significant factor affecting the hydraulics of the Liuhua reservoir.
The relationship of the karst features and gas chimneys with an abnormally large upward movement of water explains the high water cuts in many of the wells.

The field fluid movement was modeled successfully in a reservoir simulation guided by seismic attribute analyses of the fault, fracture, gas chimney, and partial dissolution zones. The resulting production-history matching of the fluid flow around the horizontal well bores confirmed the reservoir’s complex character.