Integrated data reveals offshore China structures

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This systematic integration of potential fields data with geological literature in GIS-based visual databases combines high-quality data coverage with localized geologic knowledge. This methodology promotes easy visual correlation and correction of features and trends. It also enables faster, more precise interpretation and digital presentation of results. The procedure involves:

• Correlation of published geological features to gravity and magnetic anomalies through overlay of geotiffs
• Realignment, repositioning, and extrapolation of poorly imaged, but long-recognized features, including many first-order tectonic features
• Recognition of new trends.

All the material in the "digital light table" can be selected for viewing and interpretation by toggling a switch, allowing the interpreter to spend more time looking at the relationships among data and testing ideas by constructing new combinations (Offshore, August 2001). Published results are corrected and extended, and new relationships can be derived to focus exploration.

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The GIS software promotes retention and review of results while simplifying project documentation. Tabular data is stored with all map features. The most important tool remains geologic perspective and experience.

South Atlantic reservoirs

Remarkable correlations between gravity imagery and reservoir distribution are documented in the study South Atlantic Margins Basin Analysis. The study interprets the basins in the conjugate passive margins of Brazil and West Africa. Examples from the Campos and Lower Congo basins showed respectively: sediment distribution controls by basement faulting and salt-involved compression.

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Prior work on the Oligocene fans in Brazil's Campos basin, indicated they developed from line sources associated with a sinuous, but uninterrupted, basement trend. The Samba study overlays a published fan map on a basement-related gravity isostatic anomaly.

The offset in the basement-related Badejo High (or Campos Hinge) is spatially welded to the major offset in the gravity trend. The canyons that fed the fans cluster along this offset and project back to the onshore drainage system and known reservoir provenance. The fans themselves associate with the isostatic lows (cool colors) and suggest a northeasterly extension for the thickest fan.

Salt tectonics control the sediment pathways in Angola block 16 in the Congo Fan of West Africa. Compare the published locations of the carbonate platform and rafts and silciclastic sediment infill with the gravity derivative map. The correspondence is striking. The interpreted sediment distribution pathways based purely on gravity data match within the spatial resolution of the underlying 4-km grid. The authors hypothesize that the gravity map is responding to density differences not only within the Tertiary (carbonate vs. clastic) but to large steps in the older syn-rift section that correspond to the points where rafts break away from the platform. Drainage systems are long-lived, and despite the absence of faulting or pronounced sag at the Oligo-Miocene level, the channel complexes of this age locate above older lows, as defined by the gravity trends. Basinward, the injection of salt enhances the density contrasts and makes gravity a useful prospecting tool at least down to the block and ranking level.

Southeast Asia regional structure

The tectonics of the South China Sea region both influence the petroleum systems of the surrounding seven producer nations and form a touchstone to understanding Southeast Asian geology. Broad issues remain controversial due to:

• Geologic complexity
• Conflicting paleo-magnetic data
• Spotty geochronology
• Lack of constraints from ocean spreading
• Confidential treatment of data by Asian nations.

The opening history of the South China Sea, the single largest piece of oceanic crust internal to Southeast Asia, is still subject to revision. With petroleum systems developed in the Tertiary, the pre-Tertiary tectonic history has been an academic endeavor based on onshore data. By merging data sets from both onshore and offshore, our technique highlights unforeseen pre-Tertiary source rock systems and maturation histories in the South China Sea.

The construction of continental crust involved several pre-Tertiary processes – the assembly of terrains around the Kontum massif, an overprint of several phases of Indosinian orogeny, its accretion onto China, and construction of one or more Andean-type orogens on the eastern flank in China, Vietnam, the Sunda craton, and Borneo.

The current South China Sea continental crust lies east and north of these granitic belts and appears to be largely composed of forearc orogens, including any exotic terranes that collided with the arcs. Mesozoic fossils from the South China Sea have northern affinities, rather than to the rest of Sundaland.

This and the excessive width of the arc-trench gap within the South China Sea suggest widening by means of an arc-parallel, strike-slip fault active in Late Cretaceous.

Terranes of northern origin and/or part of the Yanshan and its fore-arc were translated into the central South China Sea subparallel to the convergent margin. Much of the South China Sea, therefore, is underlain by potentially source-rock bearing forearc basins, which have yet to be factored into the South China Sea evaluations.

The area's Tertiary history is marked by rifting of this Mesozoic continental crust and seafloor spreading. The reinterpreted continent-ocean boundary (COB) has continental areas flipping into oceanic crust and vice versa along the southern margin of the Pearl River Mouth basin (PRB), moving the interpreted COB some 50 km north or south. Because the South China Sea ridge migrated with time, no single heat flow history can characterize the whole South China Sea, but the general pattern will be from low during the forearc history to high in stretched crust to very high in areas directly affected by successful sea floor spreading, then decaying during drift and sag.

The sedimentary basins along southern China and Vietnam are intimately linked to the evolution of the spreading system. The PRB and Beibuwan basins form an extensional arm of the triple junction and were de-activated at sea floor spreading. Basins on the south side of the system, offshore northwest Borneo, are built upon terranes derived from the northern margin that were translated southward by the spreading system. Strike-slip faulting and compression then further modified these. Inherited Mesozoic grains and Tertiary tectonic overprint produce a complex basement structure beneath both the northern and southern margin Tertiary basins.

Reservoir distribution

Similarly to the South Atlantic margins, basement controls on sedimentation are suggested by gravity imagery in the South East Asia Magnetics and Gravity Imaged Concepts (Seamagic) study. A comprehensive study of the South China Sea marginal basins suggests extension of a basin with a new play into an area where regional work implies a favorable heat flow history.

Mapped canyon systems (straight brown lines) and fans (mapped in bright yellow, implied in pale yellow) on gravity dip-azimuth display, offshore northwest Borneo. Note strong correlation between dip-azimuth reds and canyons.

Canyon complexes offshore northwest Borneo have been successfully explored on continental shelf and into deeper water. A distinctive gravity footprint where these complexes occur persists from the Baram Delta, with two interpreted major drainage systems, to the Philippines.

Periodic tectonic movement reactivated deep faults, localizing conduits for younger sediments. While the gravity signature corresponds to contrasts in basement structure, it reflects Tertiary sediment distribution patterns. The distinctive signature is lost beneath the Palawan thrust sheet.

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The dip azimuth displays of isostatic gravity for northwest Borneo show pronounced dip-oriented red lineaments terminating rather abruptly, where best expressed, at strike-parallel blue/blue-white areas. This gravity signature is interpreted as dip-oriented canyons and contrasting broad, strike-oriented backstops and flats that create catchment areas. Several of the interpreted canyons coincide with known channels, slumps, or point sources, and a published fan outline was used to calibrate the fan/catchment signature.

Offshore northwest Borneo, interpreted channel/canyon systems (CCS) debouch into the known Tertiary Lingan Fan and additional speculative fans. Their sources coincide with the locations of slump-scars and back-cutting shelf edges.

Pearl River fans

Similar distinct CCS patterns characterize the Tertiary Pearl River fan system, initiating at the delta front, extending across the slope and terminating at the southern basin extension. They raise the expectation for turbidite reservoirs.

The PRB is a passive margin rift system that formed over Mesozoic continental crust and inherited the pre-existing tectonic zones of weakness. Paleogene extension that formed the depocenters of the PRB occurred mainly from Late Eocene to Late Oligocene and included a component of dextral shear, possibly originating from reactivation of northwest/southeast trending strike-slip faults. The strike-slip faults appear to control some edges of the three subbasins and the uplifts, including the Shenhu Uplift boundary with the Zhu II Depression. The shear trend also seems to influence the paleo-Pearl River drainage. These basement trends appear well suited to imaging with potential fields data.

Deltaic reservoirs are the primary exploration play in the Zhu I and Zhu III subbasins while only a few wells test the edge of Zhu II, where a lack of reservoir quality sands has been perceived. The Seamagic study predicts turbidite reservoirs well outboard of the deltaic environments explored to date in a significant extension of the play beyond the Neogene thick.

The inferred restricted Paleogene-aged basin has favorable implications for source rock deposition and, with thinner total section, avoids the over-maturity of the Baiyun depocenter. An interpreted CCS suggests clastic conduits for turbidite reservoirs.

Cross-correlating

The Seamagic results were presented to industry in December 2001 and March 2002. Subsequently, the Chinese National Offshore Oil Co. announced a licensing round in the fall of 2002 and included the area of the authors' predicted turbidite play. Seismic grid and prospect maps on Cnooc's web site and a play map in their block brochure provided a publicly available interpretation of the area, well after the Seamagic results were known.

The Cnooc play map is of particular interest as it shows the sub-regional distribution of turbidite sands interpreted from Chinese proprietary seismic data. Drilling information from one exploration well (BY7-1-1, gas) and from recent ODP sites are also relevant to ground-truthing the play.

A comparison of the Seamagic potential fields prediction to the Cnooc seismic prediction shows mixed, but generally favorable, results. The western group of fans mapped by the Chinese is about 35 km up-dip of the Seamagic prediction and extends further to the west, where the BY7-1-1 well was drilled. The authors do not have access to the seismic or well data, so speculate that these may be slope fans, with additional sand accumulating downdip as basin floor fans.

There is a good correlation with the apexes of the western Cnooc fans and the Seamagic canyons. The eastern group of Cnooc fans is largely coincident with the Seamagic prediction. This close result is remarkable, given the coarse resolution (about 6 x 8 km) of the gravity data used in this study, and the speculative nature of the gravity attribute interpretation.

The Cnooc play map also showed major structures, in addition to the fans, which correlate well to structures mapped from the first derivative of isostatic gravity in the Seamagic project. The authors' bias for significant liquids in the turbidite play, as opposed to the gas found at BY7-1-1 and shallow gas anomalies mapped by Cnooc, is supported by the presence of headspace gases up to C6 at ODP sites outboard to the southeast of the play. Hydrocarbon type and turbidite sand details await the ultimate test by the drill bit.

Conclusions

Multi-data methods allow explorationists to concentrate on interpretation rather than on data gathering and research of basic regional concepts and prior work. The GIS tools simplify progress documentation and project archival. This provides good value, even if exploration efforts are unsuccessful or non-commercial, by freeing staff for the next project while still building the corporate memory.

Regional contexts are generated from readily available, inexpensive, neglected, or previously uncollated work. New correlations on basin-level features are then derived, with:

• Implications for reservoir trends and prov-enance
• Better defined heat flows and depths to basement for maturity modeling
• Improved understandings of the interplay between structure and prospectivity.

These simple methods have good predictive and correlative value based on comparisons to independent and subsequent studies and methods. The techniques have widespread application and allow prediction of additional systems in study areas.

Acknowledgement

The authors thank Getech for use of and permission to show selected potential fields images.