Roc: 'No Unpleasant Surprises' from Beibu Discovery
Roc Oil Co. Limited reported that preliminary appraisal results from the second sidetrack well drilled at the recent Wei 6-12S-1 oil discovery in Block 22/12 in the Beibu Gulf, offshore China, range from encouraging to as expected.
In terms of encouraging results, four oil-filled sands--representing a total of 22 meters gross and 16 meters net oil pay--were encountered in the Upper Sand Package, more than 45 meters downdip from the original well and outside structural closure as presently mapped. Two of these sands are not present in the discovery well.
Roc also noted that information from the two sidetrack wells, taken in conjunction with the latest analysis of data from the discovery well, confirms that the Upper Sand Package is not a single reservoir system; instead, it is made up of as many as seven distinct oil sands. Each of the oil sands has different oil-water contacts and separate oil--and, in two cases, oil and gas--columns. In six cases, individual oil columns are inferred to range in height from 40 meters to 60 meters. One column, in a five-meter sand, is inferred to be possibly as much as 180 meters.
As expected, the Middle and Lower Sand packages were encountered between 100 meters and 230 meters downdip from the discovery well and are therefore water-wet. Drill and pressure data from the two sidetracks and the discovery well established a 60-meter oil column in the Middle Sand package and infer a possible 105-meter oil column in the Lower Sand Package updip from the second sidetrack.
In addition, a deep stratigraphic exploration target proved to be a reservoir-quality sand sequence without hydrocarbons.
A more detailed discussion of the above report follows.
Since a July 26 announcement regarding the Wei 6-12S-1 exploration well in Block 22/12, the second sidetrack hole, Wei 6-12S-1Sb (1Sb) has drilled to total depth of 2,950 meters below rotary table. At midnight local time on July 31, logging operations were nearing completion.
Sidetrack Drilling Program Rationale
Following the discovery of 95 meters of net hydrocarbon pay, including 80.5 meters net oil pay, within a 477-meter gross reservoir sequence drilled by the Wei 6-12S-1 exploration well, the Block 22/12, Joint Venture immediately embarked on a two-month appraisal drilling program. As a result, two sidetracks were drilled from the original discovery well. Both achieved their main objective: the acceleration of data gathering in order to provide maximum information in regard to determining the field's commercial potential.
Overall Objectives and Results
The primary objective of the first sidetrack, Wei 6-12S-1Sa ("1Sa"), was to obtain core data from the main reservoirs encountered in the discovery well. To achieve this objective 1Sa was located about 50 meters to the east of the original hole. Five cores were cut with 100% recovery, representing 79 meters of reservoir section and additional logs and pressure data were also acquired.
The main aim of the second sidetrack well, 1Sb, was to define more precisely the vertical and lateral extent of the oil accumulations in the Upper Sand Package by penetrating that sequence more than 45 meters downdip from the discovery well. As a result of drilling 1Sb, as many as seven separate reservoir sands, all with different oil-water contacts and separate oil--and in two cases, oil and gas--columns, are now recognized in the Upper Sand Package. In six cases individual oil columns are inferred to range in height from 40 meters to 60 meters, with one column, in a five-meter sand, inferred to be possibly as much as 180 meters. This contrasts with the provisional interpretation of the initial data obtained from the discovery well that suggested that the sands seen in the Upper Sand Package might be part of a single main reservoir with a common oil water contact.
A secondary objective of 1Sb was to test a relatively deep, stratigraphic exploration target--not present in the discovery well--that proved to be a reservoir-quality sand without oil shows.
In order to achieve these dual objectives 1Sb's trajectory was designed so that it would encounter the oil reservoirs in the Middle and Lower Sand Packages drilled by the discovery well, respectively. 100 to 230 meters downdip from that well and, therefore, below their respective oil-water contacts; this proved to be the case.
The reservoir quality and continuity of the three sand packages are broadly consistent over the ca 400 meters lateral distance represented by the three reservoir penetrations. The overall section thickens downdip and additional sands, not seen in the discovery well, are present off-structure.
Preliminary Detailed Results
1Sa and 1Sb have provided drill, log, sample, and pressure data that will require months to analyze properly. However, a preliminary evaluation of the results of the three wells drilled to date provides useful insights into the nature and extent of the field. Specifically:
Upper Sand package
Although it penetrated the top of the sequence some 48 meters downdip from the discovery well, 1Sb drilled four oil sands within the Upper Sand Package. None of the sands has an oil-water contact. These sands, which collectively represent 22 meters of gross pay and 16 meters of net oil pay, are detailed below in order of increasing depth:
- The highest oil sand in 1Sb, has a gross pay of 5.5 meters, 100% net. It equates to a one-meter sand in the discovery well, where it occurs above and separate from the higher of two gas sands reported in that well. This oil sand was not specifically referred to in the May 17, 2006, Stock Exchange release that first detailed the discovery because it was considered to be immaterial. The fact that the sand has thickened downdip and an oil-water contact has not been drilled by the downdip sidetrack is encouraging. The minimum oil column inferred for this sand is 48 meters.
- The higher of two gas sands in the discovery well is also present in 1Sb, where it has a gross pay of six meters and a net pay of 5.5 meters. However, in 1Sb, this sand is oil-filled, thereby confirming the presence of a down dip oil leg associated with the highest gas sand in the discovery well. The minimum inferred oil column for this sand is 53 meters. It is further inferred that this oil leg underlies a minimum 39 meter gas column part of which was penetrated by the updip discovery well.
- An oil sand, with six meters of gross pay and about three meters of net pay, occurs within the Upper Sand Package but is absent from the updip discovery well. There is no oil-water contact in this sand in 1Sb, and the inferred oil column for the sand is 60 meters.
- The deepest oil sand encountered in 1Sb has a gross pay of 4.5 meters thick and a net pay of about two meters. This sand is also present in the discovery well where it provided a pressure reading that was considered to be possibly spurious because its implied a very long downdip oil column. This pressure information has been essentially replicated in both sidetracks thereby increasing the possibility that the oil column in this sand may be in the order of 180 meters, although further work needs to be done to more precisely evaluate this scenario.
Three sands which contain hydrocarbons in the Upper Sand Package in the discovery well are present as water-wet sands in 1Sb. Inferred vertical oil columns for these sands, up dip from 1Sb, range from 40 meters to about 60 meters.
In two cases, the inferred oil-water contacts are broadly coincident with a seismic anomaly. The anomaly suggests that such features may prove to be useful tools with regard to delineating oil and gas accumulations in and around the field.
All of the oil columns seen in the Upper Sand Package penetrated by the three wells drilled to date appear to extend below structural closure as currently mapped. Unless there is a significant unrecognized seismic velocity anomaly to the east of the three wells it would seem that there is a stratigraphic component to the trapping mechanism at the level of the Upper Sand Package. There is no seismic velocity anomaly in the immediate vicinity of the three wells drilled to date.
Middle Sand Package
This is the main oil sand drilled by the discovery well. In that well the interval has a gross thickness of 60 meters, 31 meters of which are net oil pay. The discovery well drilled through an oil-water contact--the only one penetrated by any of the three wells so far drilled in the field--that appears to be just below structural closure as currently mapped. Because 1Sb was designed to intersect the Middle Sand Package 106 meters downdip from the discovery well, it was expected to be water-bearing. That, in fact, proved to be the case.
Encouragingly, the first sidetrack, 1Sa, recovered clean oil from near the base of the main reservoir in this Middle Sand Package, which had previously been considered to be a likely transition zone. The discovery well established a 60-meter oil column in this sand updip from 1Sb.
Lower Sand Package
Although the discovery well did not drill through an oil-water contact in this sand package pressure data suggested that 1Sb would be too far (>200 meters) downdip to encounter oil in the equivalent sands. That proved to be the case. However, 1Sa cored part of this interval and a preliminary review of these data suggests that the reservoir quality is better than the test flow rate of 1,720 BOPD might indicate and that the test result was constrained by formation damage. Pressure data gathered from 1Sb infer that there is potentially a 105 meter oil column in this sand updip from that well. The lower part of this oil column appears to be below structural closure as presently mapped.
Commenting on the drilling results to date, Roc CEO John Doran stated:
"Overall, this appraisal program has provided encouraging results and no unpleasant surprises.
"The recognition of multiple oil reservoirs in the Upper Sand Package with substantial individual oil columns is a significant step forward in our understanding of the size and nature of the field.
"One of the challenges of the discovery is explaining in non-technical terms what is going on in a field which has a variety of different sands, the vast majority of which contain oil, spread out over hundreds of meters vertically within a trap that, although associated with a structure that is well defined by 3D seismic, seems to have a significant stratigraphic component, the exact extent of which is unknown. This is, however, a welcome challenge compared to explaining a dry hole.
"As a result of three months of continuous operations on a well that was expected to be have been finished within a few weeks if it had been dry, the Joint Venture has been left with three important reservoir penetrations, a wealth of subsurface data that requires detailed analysis and strong evidence that this newly discovered oil accumulation may extend beyond structural closure as presently mapped.
"If the field had a single reservoir and was confined by structural closure, assessing its true nature would have been quicker and simpler. In all probability, we would now have a fairly good idea as to the ultimate size of the accumulation. The fact that the field seems to be at least partly defined by stratigraphic elements means that although the data gathered to date is expected to confirm the commerciality of the discovery, more appraisal and/or development drilling will be required before accurate recoverable reserve numbers can be provided for the field as a whole. This is not to say that the field will necessarily prove to be huge, just that it is likely to be bigger than if it was limited by structural closure.
"A lack of specific recoverable reserve figures can be frustrating for shareholders and potential investors but, in the long run, it is considered better to admit that such precise numbers are beyond our technical reach at this particular moment rather than generate figures which may inadvertently mislead and cause a reserve downgrade in the future.
"Although exact numbers for recoverable reserves are not yet available it is possible to provide a tentative sense of direction as to the likely order of magnitude of in-place oil and the commercial potential of the field. The balance of current probabilities suggests that the oil in-place is likely to be measured in terms of many tens of millions of barrels and, if this proves to be the case, a commercial development would be anticipated."
Joint Venture Composition
The Block 22/12 Joint Venture comprises Roc Oil (China) Co. (operator, 40%), Horizon Oil Limited (30%), Petsec Energy Ltd. (25%), and Oil Australia Pty Ltd (First Australia Resources) (5%). The China National Offshore Oil Co. (CNOOC) is entitled to participate up to a 51% funding equity level in any commercial development within Block 22/12.
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