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Greg Croft Inc., 2D and 3D Seismic Interpretation


Greg Croft Inc. - March 2002 Newsletter


Oil Fields of the Gulf Region: An illustrated Atlas

This work covers the geology and reservoir parameters of the oil fields of the Arabian (Persian) Gulf region, including many of the world’s largest. This 142-page atlas contains 143 tables, 26 full-page, color illustrations and individual descriptions of 243 fields. Available now from Greg Croft Inc for $850, price includes priority mail delivery to U.S. destinations. Please contact us for other shipping arrangements.

List of Illustrations
Jurassic Source Rock Distribution Area 1 Oil Fields Index Map Bul Hanine Field, Qatar, Map Dukhan Field, Qatar, Map and Cross Section Ghawar and Abqaiq Fields, Saudi Arabia, Map Area 2 Oil Fields Index Map Area 2 Southeast Gas Fields Index Map Iran Regional Cross Sections Gachsaran Field, Iran, Map Gachsaran Field, Iran, Cross Sections Kirkuk Field Map and Cross Section Area 3 Oil Fields Index Map Burgan Field, Kuwait, Map Raudhatain Field, Kuwait, Map Safaniya/Khafji, Zuluf and Ribyan Fields, Map Area 4 Oil Fields Index Map Area 4 Trap Styles Asab Field, Abu Dhabi, Map Bab and Bu Hasa Fields, Abu Dhabi, Map and Cross Section Sajaa Field, Sharjah, Map Yibal Field, Oman, Map Zakum Field, Abu Dhabi, Map and Cross Section Area 5 Oil Fields Index Map, Oman Area 5 Oil Fields Index Map, Saudi Arabia Marmul Field, Oman, Map and Cross Section Rima Field, Oman, Map

Greg Croft Inc. – February 2002 Newsletter


Lacustrine Carbonate Oil Reservoirs

Although most of the world's carbonate rocks are marine or marginal marine deposits, lacustrine carbonates can be prolific oil reservoirs. Notable examples are the Toca Formation in Angola and the Jequia Formation in Brazil's Campos Basin. Both are Cretaceous lacustrine carbonates deposited in the chain of lakes that existed at the early stages of the opening of the South Atlantic Basin. Although the Jequia is often described as a coquina, both formations are predominantly algal.

The major lacustrine carbonate fields in Angola are Malongo West and Limba. The Toca is also productive at Takula, but most of that field's production comes from younger marine clastics. By yearend 1998, Malongo West had produced 286 million barrels and Limba had produced more than 64 million barrels. The source of these production figures, as well as those below, is the Oil and Gas Journal. The Kali Field produced 4.36 million barrels during its life, which is good for a one well field. Attempts to step out were not successful.

The Badejo, Linguado and Pampo Fields in Brazil's Campos Basin produce from the Jequia Formation. Pampo is the largest of these; it produced 30,771 barrels of oil per day from 24 wells in 1996. By yearend 1997, Pampo had produced 196 million barrels, Linguado had produced 124 million barrels, and Badejo had produced 28 million barrels.

Because exploration in lacustrine basins tends to be focussed on clastic reservoirs, many lacustrine carbonate reservoirs have not yet been discovered. The key to exploration for this play type is 3-D seismic data, which reveals the distinctive appearance of the algal mounds. Reservoir quality and well productivity are highly variable in these reservoirs. For this reason, features that have been tested with a single well that did not produce oil, or that produced it at subcommercial rates, have not necessarily been disproven. In the example of Kali Field above, if one of the dry wells had been drilled first, the successful well would probably not have been drilled.

As is the case with marine reefs and mounds, small accumulations probably greatly outnumber the major fields, but are more difficult to detect. Because lacustrine source rocks tend to be thick, rich and oil-prone, these carbonate mounds are often entirely filled with oil, with little or no water leg. For this reason, water or gas injection may be needed in order to achieve a good recovery rate.

Greg Croft Inc. – January 2002 Newsletter


Mexico's Oil Production

Mexico's oil production was about 3.1 million barrels per day in 2001, the same as in 1997. This constant level of total production gives no indication of a substantial change in Mexico's oil production from lighter to heavier crudes.

Production of light crude oil declined from 1.5 million barrels per day in 1997 to 1.2 million barrels per day in 2001. Light oil production in Mexico comes principally from the state of Tabasco and adjacent offshore areas. The light oil is produced from Cretaceous carbonate reservoirs at depths of 11,000 to over 19,000 feet. Because this is unusually deep for an oil play, many of the fields were discovered recently. The discovery of this Cretaceous play in 1972 caused dramatic growth in Mexico's oil industry. The light oil wells produce at very high rates, but fields decline within a few years. Continued exploration success until quite recently had allowed light oil production rates to be maintained by bringing additional fields on production.

Heavy oil production in Mexico comes from offshore in the Gulf of Campeche, and has increased from 1.6 million barrels per day to 1.9 million barrels per day in four years. This increase is principally due to the success of a nitrogen injection project in the giant Cantarell Field. The offshore fields are located in the same sedimentary basin as the light oil production, but produce heavy oil of around 20 degrees API. The most important reservoir in the Gulf of Campeche is a breccia of Paleocene age. Because the breccia is pervasively fractured, the reservoir has excellent vertical permeability over large intervals. Gravity drainage of the oil as a nitrogen cap is formed will allow recovery of as much as 65% of the original oil in place.

Although very costly, the nitrogen injection has increased the production of Cantarell to 1.7 million barrels per day, as compared to a maximum of 1.1 million barrels per day before. This makes Cantarell one of the most important oil fields in the world, and the most important in the Western Hemisphere, in terms of current production.

Greg Croft Inc. – December 2001 Newsletter


Gravimetric Exploration for Oil and Gas

Improvements in reflection seismic data have made it the dominant geophysical method for oil and gas exploration but, among non-seismic methods, gravimetric surveys are probably the most useful. As recently as the sixties, the discovery wells on many major oil fields were located on the basis of gravimetric data alone, and gravimetry remains a cost-effective method of studying large areas to determine where seismic efforts should be concentrated.

Existing reflection seismic data is costly or difficult to obtain for most areas, but numerous gravimetric survey data points for diverse areas around the world can be obtained from government or academic sources for little or no charge. The equipment cost for gravimetric surveys is much less than for reflection seismic surveys; a used gravimeter and GPS receiver can be obtained from a secondhand source such as R.T. Clark for about $20,000. For this reason there are many small contractors offering land gravimetric services.

Another advantage of gravimetric surveys in remote or environmentally sensitive areas is that the equipment is small and portable, making gravimetric exploration much easier to conduct than reflection seismic. Gravimetric surveys are passive, so they can be conducted in areas such as national parks with no greater environmental impact than a hiker passing through.

Screening for Applicability of Gravimetric Surveys

Like all exploration methods, gravimetry works better in some areas than in others. High-relief structures with basement involvement are more likely to offer the density contrast necessary to produce a well-defined gravity anomaly. Rift basins are almost always good candidates for gravimetric surveys, as are many features associated with strike-slip faulting. Terrain has a substantial effect on gravity data, which is why gravimetric methods work best in flatter areas. Elevation corrections are simple and effective; the more difficult problems are associated with local relief.

Modeling in two dimensions is fairly simple and can be used to determine the applicability of gravimetric data. Because gravimetric surveys do not offer the degree of detail that can be obtained from modern reflection seismic data, gravimetric surveys are usually performed where no seismic data is available. Exceptions may be made when specific questions can be answered by gravimetry based on modeling combined seismic and gravimetric interpretations. A search of publicly available gravity data is often a good way to begin work in an area or country where you do not already have a database.

Greg Croft Inc. – November 2001 Newsletter


Enhanced Oil Recovery (EOR) Issue

Thermal EOR and carbon dioxide EOR are used in numerous areas within the United States but many international opportunities remain undeveloped.

Thermal EOR usually means injecting steam. Cyclic steam injection is really a well stimulation method, since it involves using a portable steam generator to inject steam for a period of time, after which the well’s oil production is temporarily increased. Steam flood involves continuous injection of steam in some wells and continuous oil production from others, similar to a pattern waterflood. This requires a greater understanding of the reservoir than is necessary for cyclic steam.

Cyclic steam is used in many of the heavy oil fields in California and Venezuela. Steam flood is used in many fields in California, but the world’s largest steam flood project is in the Duri Field in Indonesia.

The most important screening parameter for thermal EOR is reservoir depth; the best projects are less than 1500 feet (about 450 meters) deep. Steam flood projects are almost never economical below this depth, but cyclic steam can be used down to a maximum depth of about 3000 feet (about 900 meters). Since the principal beneficial effect of thermal EOR is reduction in oil viscosity, the best project candidates have oil that is moderately to highly viscous at reservoir conditions. Paraffinic crudes typically lose viscosity more rapidly with increasing temperature than asphaltic crudes, but economical projects exist with either crude type. Since the method involves heating up the whole rock, high reservoir porosity is needed.

Carbon dioxide EOR differs from pressure maintenance by injection of natural gas or nitrogen in that carbon dioxide is very soluble in some oils. The beneficial effects of the carbon dioxide dissolution include both reducing the oil’s viscosity and increasing its volume. The difficulty with carbon dioxide injection is that it is highly corrosive under pressure in the presence of water. In practice, a carbon dioxide flood in an existing field requires replacement of all tubular goods and downhole equipment with corrosion-resistant parts. This same corrosive effect provides the benefit of improving the permeability of carbonate reservoirs, similar to an acid stimulation.

Most of the carbon dioxide used for EOR in the United States comes from southern Colorado and is transported by pipeline to projects in West Texas, Oklahoma, western Colorado and Wyoming.

Screening parameters for carbon dioxide EOR include reservoir depth, oil gravity and reservoir geometry. Carbon dioxide floods work best when the carbon dioxide is a critical fluid, which implies depths greater than 2000 feet (about 600 meters). Carbon dioxide is more soluble in lighter oils, so another screening parameter is that the oil should be 33 degrees API or lighter. Laminated reservoirs are better than massive reservoirs for carbon dioxide flood because carbon dioxide is much lighter than oil and tends to channel along the top of massive reservoirs.

Greg Croft Inc. – October 2001 Newsletter


Temblor Formation Discoveries Increase Kern County Gas Production

Recent discoveries in the Temblor sandstone of Miocene age have caused nonassociated gas production in Kern County, California to increase 61% from January 2001 to March 2001, but since March there has been a two month decline. The Temblor is a marine sand at the base of the Monterey Formation, a diatomaceous shale that is the source rock for nearly all of California’s oil – currently 800,000 barrels per day. In the deep San Joaquin Basin, this oil-prone source is buried deeply enough to generate gas and condensate. The Temblor retains enough porosity at 20,000 feet to be a commercial reservoir. Greg Croft Inc has 280 acres of US federal leases in the deep basin area.

Kern County, California, Nonassociated Gas Production

May 200120.8 Mmcfd.
April 200123.1
March 200124.6
February 200122.9
January 200115.3

Source: California Division of Oil, Gas and Geothermal Resources

Middle East Oil Production Trends

Middle East oil production has again reached the levels of the seventies, but the relative importance of the major plays has changed. First half-2001 oil production from each of five major plays is compared with 1977 levels below. The first play is the Jurassic platform carbonates of Saudi Arabia, Bahrain, Qatar and the UAE. The second play is the Tertiary and Cretaceous fractured limestones associated with the Zagros Fold Belt in Iran and Iraq. The third play is the Cretaceous sandstone reservoirs around the head of the Persian Gulf, along with some deeper Jurassic and Cretaceous carbonate reservoirs. The fourth play is the rudistid mounds within the Cretaceous carbonates of Oman, Saudi Arabia, and the UAE. Iran’s Sirri offshore fields are also part of this play. The last of the five plays is the recently discovered rift basin play of Yemen.

The dramatic growth in Play 4 is due to numerous pattern waterfloods, as well as completion of the major Upper Zakum and Shaybah projects. The plays with strong natural water drives (Play 3), fractured reservoirs (Play 2), or that can be easily waterflooded (Play 1) require less capital investment per daily barrel of production, so these plays were already intensively developed by the mid-seventies. Future oil production growth will come from Saudi Arabia’s Qatif and Khurais developments (Play 1), deeper exploration (Play 2) and continuing waterflood development and exploration (Play 4).

Sources: US DOE/EIA, Oil and Gas Journal
19772001 (first halfChange
Play 1: Jurassic Carbonates8.7 million BOPD6.1 million BOPD-30%
Play 2: Asmari and Bangestan6.15.4-11%
Play 3: Cretaceous Sandstone5.14.9-4%
Play 4: Thamama Equivalent1.83.6+100%
Play 5: Yemen Rift00.5NA
Total (5 plays)21.7 million BOPD20.5 million BOPD-5%



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