PRIME, POOR QUALITY SEEN FOR PROSPECTS IN TARIM BASIN OF CHINA

China's first Tarim basin oil and gas tender includes an unusual mix of prime and poor prospects, a U.S. geological and engineering company has found. Of five Tarim basin blocks offered by China National Petroleum Co. (CNPC) in a bidding round that began last Mar. 1, Gustavson Associates Inc., Boulder, Colo., rates the geological characteristics of the most prospective tract as variable but generally positive. (For site of the basin see map, OGJ, Apr. 12, p. 36.)

Oct. 18, 1993

6 min read

China's first Tarim basin oil and gas tender includes an unusual mix of prime and poor prospects, a U.S. geological and engineering company has found.

Of five Tarim basin blocks offered by China National Petroleum Co. (CNPC) in a bidding round that began last Mar. 1, Gustavson Associates Inc., Boulder, Colo., rates the geological characteristics of the most prospective tract as variable but generally positive. (For site of the basin see map, OGJ, Apr. 12, p. 36.)

Gustavson said another block is mediocre but probably will attract bidders, while the remaining three tracts should be considered very high risk.

Those findings appear in a nonexclusive study of the basin by John Weiner, Gustavson's chief geologist, based mainly on public data.

"It is highly unusual for anyone to offer such widely differing concession blocks," said John B. Gustavson, president of the firm. "As large international companies learn about the drastic differences in source rocks, reservoirs, traps, seals, drilling depths, and discoveries, they will compete aggressively for the one or two favorable blocks while the others go begging."

That strategy by big companies or groups of majors could allow groups of independent oil companies a chance to win concessions on some higher risk acreage, he said.

CNPC's first Tarim tender is offering concessions on 72,730 sq km of acreage divided into these blocks: Yatongguzi, Xiaoertang, Qiemo, Tulabei, and Washixia (see map, OGJ, Oct. 11, p. 23). Gustavson declined to disclose which tracts appear to be most promising.

RELIABILITY OF DATA

Weiner said Gustavson rated each tract based on the presence or absence of lower Paleozoic source rock, suitable reservoir rocks of several geological ages, and extent of structuring or trap formation.

Weiner reached his conclusions by piecing together information from many sources. He cautioned reliability is suspect for much of the Chinese data on which the study is based.

For example, various Chinese geological reports place Tarim discoveries at different locales, and field names van, from source to source. Fields in some instances are known only from value references or rumors, so some might not exist. Conversely, some real fields might not appear on available maps.

"I don't think there is a deliberate plan to mislead companies," Weiner said. "CNPC officials know where everything is, but the literature is unclear."

But, he said, "Even Chinese officials might not yet be sure how big some Tarim fields are."

Information also is spotty about the first five Tarim basin blocks offered for concessions. For example, available information indicates blocks included in the first round are only sparsely explored by drilling, if at all.

"We're not sure, but some of the blocks might not have any wells on them whatsoever," Weiner said.

Gustavson's study estimated well costs in the area at $12-16 million for a 6,000 m Paleozoic test, partly because of the high costs of moving equipment into the remote region.

The company could not quantify current Tarim oil and gas production or determine which fields are on line. And it found scant evidence of oil and gas transportation infrastructure in the first round tracts.

One of the few certainties: The Qiemo block is among closest in the group to China's 1989 Tazhong oil and gas discovery on the Central Tarim uplift.

GEOLOGIC INDICATORS

Despite the limitations of available data, Gustavson concluded that quality and maturity range widely among the basin's potential source rocks.

Based on discoveries to date, lower Paleozoic Cambro-Ordovician gray to black carbonaceous marine shales appear to have the best oil and gas potential in the basin. Carboniferous black shales and carbonates also might have potential but have not yet been linked to known oil and gas accumulations, Gustavson said.

Upper Triassic-middle Jurassic lacustrine mudstones and oil prone coals are the main sources of traps in the Kuqa and Southwest depressions but appear to be less important compared with Cambro-Ordovician strata, Cenozoic source rocks likely are of only localized importance.

Gustavson counted three main types of known or expected reservoir rocks in the basin:

Clastic sandstone and conglomerate reservoirs of early Silurian, Devonian, Carboniferous, Permian, early and middle Jurassic, Cretaceous, and Tertiary ages.
Carbonate, mostly marine sedimentary sequences of the upper Silurian, Cambrian, Ordovician, Carboniferous, lower Permian, Cretaceous, and Paleogene.
Weathered layer reservoirs resulting from frequent uplifts and long exposures of thick zones, especially in the North Tarim and Central Tarim uplifts where extensive weathering and leaching has occurred to carbonates now lying beneath an unconformity.

Gustavson said Carbonate reservoirs, generally fractured, fossiliferous, and locally cavernous, karstified, or microcavernous, partially contain skeletal grains in algal mats and bio-herm reef complexes. The most important weathered layer reservoirs associated with unconformities are beneath Devonian, Carboniferous, and Triassic sections and between Mesozoic and Cenozoic strata.

TRAPS AND SEALS

Gustavson found that the most important Tarim basin structural traps are anticlines. More than 200 anticlines have been mapped in the basin-137 on the surface and 68 in the subsurface.

The most obvious anticlines are compressional folds near mountain fronts bordering the basin.

Broad platform anticlines, including some with areas reported to exceed 1,000 sq km, are present in regions such as the Central uplift. Reverse fault related traps, which sometimes caused folding of overlying strata, are important in Paleozoic sections of the North Tarim uplift. Mesozoic anticlines draped over Paleozoic erosional and/or structural highs also are important in the North Tarim uplift.

Buried hill stratigraphic traps, partially controlled by an adjacent unconformity, have shown their importance in the North Tarim uplift. Also, truncated sandstone reservoirs in the area sealed by mudstones lying above the unconformity have oil and gas pays.

Gustavson said other types of stratigraphic traps in clastic rocks-submarine fans or river channels, for example-can be expected in varied marine and lacustrine deposits common to the basin. Reefs postulated in the Carboniferous-lower Permian also could bear hydrocarbons, but that is not confirmed.

The basin's Paleozoic seals include lower Cambrian black shales, middle Cambrian gypsum beds and clayey shales, lower Silurian clayey shales, and Carboniferous gypsum beds and clayey shales. Such Paleozoic seals in the Cambrian and Silurian could be somewhat regional, but Gustavson said there is a doubt about the regional nature of upper Paleozoic strata.

Gustavson also concluded adequate seals probably are not a problem in the basin's Mesozoic-Cenozoic formations, and shales and mudstones should be adequate seals in the Jurassic, upper Cretaceous, and Paloegene-Miocene.