SOUTH KOREA: AN OVERLOOKED HYDROCARBON PROVINCE?

David C. Sturt, Nigel A. Quinton
Dragon Oil plc
Harpenden, England

Forthcoming well about to spud off South Korea (Fig. 1) is planned to test the potential of an essentially unexplored rift basin.

Should the well prove successful, it could open up a major new hydrocarbon producing province with substantial reserves. The well is being drilled in Block Y and will be operated by KR (Far East) Ltd. (Kirkland, a subsidiary of Dragon Oil plc). The well has the potential to considerably alter the industry, s perception of South Korea's potential as a hydrocarbon province.

The block covers 7,000 sq km. It is divided into two parts and lies about 150 km south of the Korean peninsula close to Cheju Island. The larger Northern Block overlies the Fukue basin (Fig. 1), which is a linear graben system oriented northeast-southwest.

The Fukue basin is equivalent in size to the South Viking graben of the North Sea (Fig. 2) and the existence of a restricted marine source rock and play types at several stratigraphic levels helps to reinforce such a comparison. Seismic mapping has revealed five prospects with combined most likely reserves of more than I billion bbl.

The block was initially awarded to Hadson Energy in 1986 in equal partnership with Korea Petroleum Development Corp. (Pedco). During the early stages, the exploration effort focused on the Southern Block. Two wells were drilled, 1 Okdom and 1 Geobuk, but neither encountered the Eocene source interval that is believed to source the Chinese discoveries to the south, and both were dry holes.

After assuming operatorship in 1991, Kirkland refocused exploration on the northern block. The critical exploration steps there taken. The first was to review more thoroughly the available well data from the Japanese wells 1 and 1A Fukue, which prove the existence of a thick Eocene source interval known as the Chinu formation together with excellent Miocene sandstone reservoirs.

The second was to conduct a detailed seismic review involving reprocessing, seismic stratigraphic analysis, and seismic inversion.

The license is covered by more than 6,500 km of seismic data, all of which is oriented north-northwest/south-southeast (dip) and west-southwest/east*-northeast (strike). The original (1984) 4 by 6 km regional grid has been progressively infilled between 1987 and 1992, and over the key prospects the dipline spacing is 500 m.

Over the prospect to be drilled, all 1980s data have been reprocessed to the same high standard as the latest infill grids. As a result, structural control is generally very good.

The third component of Kirkland's analysis involved basin analysis and the recognition of a marked variation in subsidence history from northeast to southwest with-in the Fukue basin (Fig. 3).

BASIN EVOLUTION

The Fukue basin initiated in the Eocene (Fig. 4) as a result of extension induced as the Eurasian plate overrode the Oceanic Pacific plate, and formed a narrow deep depression 300 km long incorporating several distinct depocenters.

Fault controlled subsidence continued through the Oligocene. Evidence suggests that during this time lacustrine conditions prevailed in the basin centers. At the top of the Oligocene sequence, sporadic occurrences of marine brackish taxa interbedded with coaly beds indicate that marginal marine conditions had returned.

A second phase of rifting during the early-middle Miocene is evident from seismic data (Fig. 4), although an element of thermal subsidence as a result of the previous rifting is also apparent as seismic reflectors begin to extend over the intra-basin highs and onto the basin mar-ins. The faulting at this time appears to reactivate the pre-existing Eocene faults.

Immediately following the rift sequence, a regional seal of some 200 m of shale with interbedded coal was formed, as seen in wells 1 and IA Fukue.

Thermal subsidence continued from the Upper Miocene to the present day interrupted by two periods of local inversion at 10 million years ago and 5 million years ago, respectively.

SOURCE AND MIGRATION

During the early 1970s a brief period of exploration activity saw several wells drilled in the northern portion of the Fukue basin by Shell.

All the wells have reported hydrocarbon shows. Two well I and IA Fukue, flowed small quantities of oil and gas, providing good evidence of a working source.

Based on well data, modeling studies have shown that the northeast part of the basin suffers from a timing problem, with generation and migration taking place prior to the latest fault movement in the area. All traps drilled in this area are fault controlled, and such movement may have been critical to trap integrity.

This history is admirably demonstrated by the Fukue wells. The Sayori sandstone exhibits dead oil shows throughout, indicating a breached trap. A small amount of late generation has recharged the trap with gas, as evidenced by drillstem tests 4 and 5 in 1A Fukue, which flowed 3.1 MMcfd of gas from a 15 m gas zone clearly identifiable on logs.

In the southwest of the Fukue basin within Block Y, faults are less pervasive and a considerable greater thickness of Late Miocene and Pliocene sediment has been deposited (Fig. 3). Potential source kitchens are currently at their maximum state of maturity, and a greater amount of hydrocarbon expulsion can be expected within the last 5 million years.

Expulsion has been modeled for the Eocene Chinu formation and assumes an original average total organic carbon content of 5% and a net to gross of 50% which yields 12 million tons of hydrocarbon for each square kilometer of mature source kitchen. Allowing for variation in depth and maturity, we calculate that some 7,560 million tons of petroleum, i.e. more than 50 million bbl, has been generated and expelled from the area of mature source kitchen within the block.

The basin therefore has the potential to expel many times the amount of oil required to char-e the traps in the basin, even allowing for the degree of uncertainty inherent in such calculations. As importantly, some 50% of this generation and expulsion has occurred in the last 5 million years, subsequent to the latest structural movements.

RESERVOIR AND SEALS

The main reservoir rocks are believed to be lower-middle Miocene sandstones. Both 1 Domi and 1 Fukue (Fig. 5) penetrated excellent quality clean sandstones of fluviodeltaic origin in two intervals of the lower-middle Miocene:

The Sayori sandstone, which has a net sand thickness of 173-280 m in the Fukue basin, and
The Basal Miocene sandstone, which has a net sand thickness of 50-60 m.

Log porosities within these sandstones exceed 30%, although the greater depth of burial in Block Y implies that porosities in the range of 18-281, are more likely in the mapped prospects.

Other potential reservoirs are the lower to middle Oligocene syn-rift sandstones (Kasago formation). These sandstones are part of the restricted rift sequence and are expected to be less laterally extensive than the Miocene sandstones. Net sandstone thickness from this interval in 1 Domi is some 125 m (in 10-20 m thick units), with porosities in the range of 18-26%.

At least two regional seals can be recognized in the Fukue basin; these are the Upper and Lower Claystone members of the Sayori formation (Fig. 5).

The Lower Claystone member (LCM) is a marine shale that has a thickness of 221 m in I Fukue and 77 m in 1 Domi, contains infrequent sandstone interbeds, and overlies the Basal Miocene sandstone.
The Upper Claystone member (UCM) comprises shales, silts, and coals laid down in marginal marine conditions. It is more than 200 m thick in both 1 Domi and I Fukue. The UCM provides a top-seal to the Sayori sandstone and also to deeper reservoirs where local erosion causes these to subcrop the UCM.

Shales within the late Miocene sequence are believed to provide a third regional or semi-regional seal. Although sandy in 1 Fukue, this sequence is thicker in 1 Domi and contains several shale intervals up to domi thick. From seismic it is clear that the Late Miocene in Block Y is at least double the thickness drilled in 1 Domi, and seismic character supports the hypothesis that thick interbedded sands and shales are present.

The 1 Domi exhibits good potential for intraformational seals within the Mid-Oligocene to Early Miocene section, providing the potential for stacked reservoirs within structural traps, particularly likely to be successful in prospects exhibiting four way dip closure.

WELL LOCATION

The forthcoming well Dragon I is to test a classic rift basin structure. The trap comprises a north dipping, rotated fault block (Fig. 6). The structure formed progressively from the Eocene onward but has not been modified since the Late Miocene.

Closure is mapped from basement up to the Green Horizon (base 'Upper Claystone' seal) and is at its maximum areal extent at the Yellow horizon (top of the primary reservoir objective).

The well location is shown on Fig. 7, with the well being drilled at a 38' angle from 1,100 m to follow a trajectory down the back of the main fault. This will maximize the possibility of encountering Oligocene and Eocene sands close to the structural high point.

A seismic sequence stratigraphic analysis has assisted in predicting the reservoir/seal relationship across the structure (Fig. 8). A well/seismic tie at 1 Domi supports the interpretation that the basal Miocene and Sayori sandstones occur immediately above sequence boundaries. These sandy units are overlain by finer, sealing sediments.

At the prospect three seismic sequences can be identified across the structure (Fig. 8). Within each sequence sandstones are believed to lie above sequence boundaries S1, S2, and S3. The sands resting on S1 and S2 appear to lap out onto the main upthrown fault block.

The deepest sequence boundary (S3) lies 100-150 milliseconds below the Yellow horizon, which marks the top of an opaque interval, analogous to the seismic character of the Sayori sandstone at 1 Domi.

This interval is interpreted to be a syn-tectonic sand package that represents either the Sayori or basal Miocene sandstones. Either way, the target sandstones offer a high quality reservoir.

CONCLUSION

The Fukue basin has been overlooked by the industry in recent years.

The southwestern unexplored part of the basin is currently at its deepest burial, thus offering a greater chance of exploration success than the drilled area to the northeast, where excellent quality source, reservoir, and seal rocks are proven to be present.

The license area contains several identified prospects of similar size to the Dragon prospect, so success in the forthcoming well will open what could prove to be a very significant hydrocarbon province.