What's new in exploration

	Vol. 225 No. 10
PERRY A. FISCHER, EDITOR

Crude biodegradation. Biodegradation of crude in petroleum reservoirs is an important exploration risk with major economic consequences. World Oil presented a paper by (Alan Yu, et al. April, 2002) that attempted to quantify that risk through integration of temperature and hydrocarbon-charge histories modeling. At and near the surface, this alteration process by aerobic degradation of petroleum is well documented. It has long been thought that oxygen- and nutrient-bearing surface waters percolate into reservoirs, and that this flow was necessary for in situ oil biodegradation.

However, biodegraded oils are found in reservoirs where aerobic conditions are unlikely. This, together with the recent identification of several anaerobic microorganisms in oil fields and the discovery of anaerobic oil-biodegradation mechanisms, means that anaerobic biodegradation processes could also be responsible. Such anaerobic degradation in deep petroleum reservoirs has thus far been strongly contested. Moreover, no single organism has yet been isolated that has been proven to degrade hydrocarbons under the conditions found in deep petroleum reservoirs.

In the Sept. 16 issue of Nature, authors Carolyn Aitken and D. M. Jones from the School of Civil Engineering and Geosciences, University of Newcastle upon Tyne, and S. R. Larter from the Dept. of Geology and Geophysics, University of Calgary, Canada, presented Anaerobic hydrocarbon biodegradation in deep subsurface oil reservoirs. The research reports on the isolation of metabolites, which are indicative of anaerobic hydrocarbon degradation, from a large fraction of 77 degraded oil samples. These were from both marine and lacustrine sources from around the world, including the vast Canadian tar sands. Their results suggest that anaerobic hydrocarbon degradation is a common process in biodegraded subsurface oil reservoirs.

Where's that rig? Here is a report from MMS on some of the damage from Hurricane Ivan. The semisubmersible rig, Deepwater Nautilus went missing. The crew was previously evacuated, and the well was secured. Transocean searched for the rig, which was moored to the seafloor with anchors and related equipment, about 160 mi south of Mobile, Alabama, in the Gulf of Mexico. A day later, the rig was found some 70 mi northeast of its original location.

Other mayhem includes: four other mobile drilling units were set adrift and subsequently located, one is listing about 3°; a rig derrick is leaning over the edge of the Spar on which it was installed, another rig derrick on a Spar is missing; seven fixed platforms were destroyed, another is leaning; two Spars have extensive damage; four platforms have extensive damage; 13 pipeline leaks were reported, one of which resulted in a fire. And this was just the preliminary assessment. However, in perspective, this damage is out of 4,000 platforms and 117 rigs in the Gulf.

Venezuelan offshore tender. Venezuela's energy ministry has pre-qualified 21 companies to participate in a tender for gas exploration contracts on seven blocks in the Gulf of Venezuela and in northern Falcón state. The ministry is preparing a data pack (available by October 1) with seismic information that will be sold to companies for $100,000-$250,000. The ministry previously indicated that the blocks would be awarded toward year end. The pre-qualifiers are reportedly ExxonMobil, Shell, Statoil, Total, Eni, Conoco-Phillips, Repsol-YPF, Petrobras, Petrocanada, BP, Amerada Hess, CNPC, America Limited, Pluspetrol, Inepetrol, Teikoku, Vinccler, Zonatrah, BG Group, BHP Billiton and Marathon Oil, according to news source Multipuerta and newspaper El Universal.

Noteworthy discoveries. The small Norwegian oil company DNO finished drilling the Nabrajah-2 in Yemen's Block 43, at a TD of 8,350 ft. The well confirmed oil in the Qishn interval, which is the main reservoir. However, well logs indicated that at there was hydrocarbon further downhole, at TD, in the lower basement. A subsequent production test in the basement interval flowed 3,000 boe per day (15.2 MMcfgd and 313 bpd condensate). Nabrajah-2 is the first of two appraisals to determine the size of the Nabrajah oil discovery reported earlier this year. The second appraisal, drilling in the western area of Tasour field in Block 32, spudded on September 2. When the Nabrajah-2 test is combined with the results from earlier Nabrajah-1 basement tests, there are indications that the hydrocarbon discovery could be of significant size. DNO is the operator of both Block 43 (56.67%) and Block 32 (38.95%).

Basement is generally considered to be composed of igneous and metamorphic rock and in some cases, other types of very low intergranular-porosity rock, with very different properties from the overlying rock. It is a loosely defined term that usually means the surface below which there is no current exploration interest, since there are no sediments at or below basement. However, hydrocarbons are sometimes found in basement, possibly due to downward expulsion or some other mechanism. Oil is produced from basement in both Block 14 (operated by Nexen), some 25 mi from the Nabrajah structure, and Block 10 (operated by Total).

ChevronTexaco hit with its Jack-1 deepwater Gulf of Mexico exploratory well. The Jack prospect is about 270 mi southwest of New Orleans on Walker Ridge Block 759, and encountered more than 350 ft of net pay oil sands. The well was drilled to about 29,000-ft TD, and is located in 6,965 ft of water. The company expects to conduct more drilling to delineate the discovery, which is in the emerging lower Tertiary trend.

“We have had a greater than 50% success rate in our 16 wildcat wells since late 2001,” said Ray Wilcox, vice president of ChevronTexaco. ChevronTexaco is the operator with a 50% working interest in the discovery, Devon has 25% and EnCana has the remaining 25%.

ChevronTexaco also had a significant gas discovery with its Wheatstone-1 well, located offshore 110 mi west-northwest of Dampier in Western Australia, in retention lease WA-17-R. A production test flowed 54 MMMcfgd, constrained by rig equipment. The lease is held 100% by ChevronTexaco affiliates. Wheatstone-1 was drilled to 11,096-ft TVD using the semisubmersible Transocean SEDCO 703, in 700-ft water depths. The well encountered about 175 ft of net gas sands in the Jurassic Tithionian and Triassic Mungaroo AA sands within a 413-ft hydrocarbon column.