A 3D seismic survey over the Intisar E field in the Ajdabiya Trough of the Sirte Basin, Libya, revealed a channel-like feature in Eocene carbonates that wraps around the pinnacle reef that contains the reservoir. We have used coherence, curvature, and spectral decomposition seismic attributes to determine the morphology and gray-level co-occurrence matrix attributes to define seismic facies within the feature. These indicated that the channel originated by submarine scouring caused by downslope movement of turbidity currents. Erosion was followed by the deposition of successive layers of carbonate debris in the channel. Stratigraphic correlations with the adjacent pinnacle reef revealed that the channel was cut during the late stage of reef growth, and a second channel formed after the Intisar E reef ceased to grow. Differences in seafloor elevation over the reef probably diverted turbidity currents so channels were not cut into the reef, breaching the reservoir. This interpreted geologic history may explain why some pinnacle reefs in the Intisar complex contained giant reservoirs, whereas others were barren.

In the central-eastern Sirt Basin, enigmatic Intisar domal structures host significant hydrocarbon accumulations. These structures have been commonly interpreted as pinnacle reefs/bioherms occurring in the open-marine basinal environment. Generally, pinnacle reefs/bioherms are mainly characterized by in situ carbonates. The current study challenges the Intisar pinnacle reef/bioherm model by examining one of the domal structures in terms of biostratigraphy, microfacies and depositional environment. These structures were dated using larger benthic foraminifera, which yielded a Middle to Late Paleocene age (Selandian–Early Thanetian). Thirteen microfacies types representing different carbonate ramp environments ranging from outer ramp to inner ramp, were defined. Outer ramp deposits have been observed adjacent to the domal structure, represented mainly by wackestone with small benthic and planktonic foraminifera. The outer ramp deposits are most likely isochronous to the domal structures. The lower part of the domal structures is composed mainly of foraminiferal–algal–echinodermal packstones. The upper part is characterized by foraminiferal–algal–echinodermal packstones with intercalated microbialite–coral boundstones. The euphotic inner ramp deposits are preserved on the crest of the domal structure, consisting of grainstone and packstone rich in Glomalveolina. As a result of this study, the Intisar domal structures are seen as erosional relics of a carbonate ramp and no evidence for pinnacle reef/bioherm model was found.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThe Intisar 'D' reservoir is a carbonate reef of Paleocene origin with no appreciable flow barrier. This is an under saturated reservoir with an oil of 40 °API and solution gas oil ratio of 595 SCF/STB. The reservoir, having 452 ft of net pay, is at a datum of 9,000 ftSS with an initial OOIP of 1.76 billion stock tank barrels and initial reservoir pressure of 4,257 psia. A series of very successful reservoir management strategies have been applied in order to optimize and increase the production capacity and reserves. So far 69.2 % (1.2 MMSTB) of OOIP has been recovered mainly due to implementation of an EOR project.cumulative oil production was 1,218 MMSTB, of which 823 MMSTB were driven due to gas injection as secondary and tertiary recovery methods while 395 MMSTB were produced due to waterflooding. This is one of the biggest EOR processes worldwide. This paper discusses the strategy used to produce this reservoir and also to compute the produced oil recovery from the different drive mechanism employed.

The Intisar ‘D’ reservoir is a carbonate reef of Paleocene origin with no appreciable flow barrier. This is an under saturated reservoir with an oil of 40 °API and solution gas oil ratio of 595 SCF/STB. The reservoir, having 452 ft of net pay, is at a datum of 9,000 ftSS with an initial OOIP of 1.76 billion stock tank barrels and initial reservoir pressure of 4,257 psia. A series of very successful reservoir management strategies have been applied in order to optimize and increase the production capacity and reserves. So far 69.2 % (1.2 MMSTB) of OOIP has been recovered mainly due to implementation of an EOR project. From the commencement of production in 1969 up to 1981, Intisar ‘D’ was under bottom water injection for reservoir pressure maintenance and crestal hydrocarbon gas injection for pressure maintenance, gas conservation and enhanced oil recovery; then water injection was halted, but gas injection has been continued successfully until now. Against common knowledge, oil recovery to gas (immiscible) was more effective than water flooding. For analysis purposes the reservoir was divided in two portions: Water Swept Zone (WSZ, from the base of the reservoir up to the level of WOC in 1981) and Gas Swept Zone (GSZ, from the level of WOC in 1981 to the top of the reservoir) to separately evaluate the displacement sweep efficiencies. Gas injection at the crest of the reservoir has been an extremely successful drive mechanism, although this process has been immiscible most of the time. To December 2005 cumulative oil production was 1,218 MMSTB, of which 823 MMSTB were driven due to gas injection as secondary and tertiary recovery methods while 395 MMSTB were produced due to waterflooding. This is one of the biggest EOR processes worldwide. This paper discusses the strategy used to produce this reservoir and also to compute the produced oil recovery from the different drive mechanism employed. Introduction The Intisar "D" reef reservoir is a carbonate bioherm reef of Paleocene origin and which lies within Concession 103, where seismic operations were begun in 1967; It is located in the east-central part of the Great Socialist Peoples Libyan Arab Jamahiriyah (see Figure 1), 220 miles (354 Km) south of Benghazi and 525 (845 Km) east-southeast of Tripoli and it is part of the prolific Sirte Basin. Intisar ‘A’, ‘C’ and ‘D’ belong to the same concession 103; Intisar ‘A’ was first discovered in April 1967 with first well A1 at a well depth of 9,417 ft; the third exploratory well drilled in concession 103 discovered Intisar ‘C’ in September 1967 and fourth well discovered Intisar ‘D’ .