Data published about proved reserves in the northern Gulf of Mexico for oil pools have reserves ranging from 1,000 bbl to 143 million bbl, and data published about proved reserves in gas pools there have reserves ranging from 2 MMcf to 718 bcf.
Pools are combinations of all reservoirs in a field by the same age that occur in a play. In any area with high capital expenses and high drilling risk, it is important to avoid areas with marginal or no oil and gas reserves.
Much exploration effort should be involved in examining reservoir properties such as depth of production, thickness of pay zones, productive areal extent, porosity, formation pressure, oil gravity, gas specific gravity, and water saturation in an effort to determine if and how they affect the voloumes to be recovered.
A large amount of deepwater exploration has taken place in the US gulf with no data published about reservoir properties. Not all wells in the Gulf Coast had oil and gas reserve data published, so only those properties with reserves data available are reviewed here.
Averages and ranges of published reservoir properties with and without reserves were examined, and little variation was found in the property values. Permeability was not considered here as little data have been released. To assist in the accompanying figures, only property values with data were plotted, so the data may appear to have gaps.
Field sizes
The difference between published information about small oil and large oil reserves and small gas and large gas reserves is extremely important in the search for reserves.
To demonstrate the difference, oil reserves were separated into 16 intervals of 5 million bbl each.
No data were reported between the 65 million bbl to 75 million bbl intervals and between the 80 million bbl to 100 million bbl intervals.
The smallest reserve size of zero to 5 million bbl had 28% of the reserves and 93% of the pools. With so many pools in the smaller reserves, the average size was examined. Average size of the oil reserves from the smallest interval to the largest ranged from 762,000 bbl to 143 million bbl (Fig. 1), increasing as the size of the oil reserves increases while the number of pools decreases.
The average data for the 140-150 million bbl interval are much larger than the average data for the zero to 5 million bbl interval.
Gas reserves were separated into 18 intervals of 20 bcf each. No data were reported between 280 and 360 bcf or between 420 and 700 bcf.
The smallest reserve size had 27% of the total reserves and 78% of the pools. Average size of the gas reserves from the smallest interval to the largest ranged from 5 bcf to 718 bcf (Fig. 1), increasing as the size of the gas reserves increased, while the number of pools decreased.
The average data for the 700-720 bcf interval is much larger than the average data for the zero to 20 bcf interval.
Formation age
A quick review of the published oil reserves with reservoir properties by ages showed 51% of oil reserves were Pleistocene, 32% were Pliocene, 17% were Miocene, and less than 1% were combined Oligocene and Jurassic.
Forty-three percent of gas reserves were Pleistocene, 17% were Pliocene, 34% were Miocene, 5% were Jurassic, and less than 1% were Oligocene.
Fig. 2 shows the percent of total oil reserves and total gas reserves by 10 age units. The largest groups of gas reserves were in Lower Pleistocene reservoirs followed by Middle Miocene reservoirs.
Reservoir depth
Depths of much of the oil and gas production have increased since the property values were released.
Even though the deepest well in the Gulf of Mexico was drilled to more than 38,000 ft, property values in data available to the author have ranged only from depths of 950 ft to 22,600 ft with an average depth of 7,817 ft.
Fig. 3 shows the share of oil and gas reserves by depth of production separated into 23 intervals of 1,000 ft each. Optimum depth of oil reserves shows 67% of the reserves were at seven depth intervals from 6,000 ft to 12,999 ft.
Nineteen percent of the oil reserves were at depths of less than 6,000 ft, and 14% were deeper than 13,000 ft. Optimum depth of gas reserves shows 65% of the reserves were at seven depth intervals from 6,000 to 12,999 ft. Nineteen percent of the gas reserves were at six depths of less than 6,000 ft, and 16% of the gas reserves were at 13,000 ft or deeper.
Pay thickness
Pay thicknesses with oil and gas reserves reported that have been published ranged from 2 ft to 2,258 ft with an average pay of 48 ft. Fig. 4 shows the share of oil and gas reserves by pay thickness separated into 16 intervals of 20 ft thickness.
Sixty-one percent of the oil reserves were in five thickness intervals of 40 ft, 60 ft, 80 ft, 100 ft, and 120 ft. Fifteen percent of the oil reserves were between zero and 20 ft thickness intervals, and 24% of the oil reserves were in the 140 ft to 360 ft thickness intervals.
Sixty-one percent of the gas reserves were in the 20 ft, 40 ft, 60 ft, and 80 ft thickness intervals. Gas reserves below 20 ft in thickness were 5% of the total, and gas reserves in thickness intervals above 100 ft were 33% of the total.
Areal extent
Productive areas with oil and gas reserves that have been published ranged from 5 acres to 26,404 acres with an average area of 1,073 acres.
Fig. 5 shows the share of oil and gas reserves separated into 17 intervals of productive areas in thousands of acres.
Eighty-eight percent of the oil reserves were in reservoirs that covered no more than 3,999 acres. All the remaining reserves, 12%, were in intervals from 4,000 acres to 26,000 acres.
Eighty-six percent of the gas reserves were in six intervals no larger than 3,999 acres. Acreage in 13 intervals between 4,000 acres and 26,000 acres had 14% of the total gas reserves.
Reservoir porosity
Porosity values that have been published ranged from 5% to 39% and averaged 29%. Fig. 6 shows the share of oil and gas reserves divided into 13 intervals of 2% porosity from 4% to 38%.
Eighty-five percent of the oil reserves were in four intervals from 26% to below 32% porosity. Oil reserves in intervals with porosity smaller than 26% contained 10% of the total oil reserves, and oil reserves in porosity intervals of more than 32% had 5% of the total oil reserves.
Gas reserves in porosity intervals from 26% to below 32% had 72% of the gas reserves. Porosity lower than 26% had 23% of the gas reserves, and porosity 32% or higher had 5% of the gas reserves.
Formation pressure
Published formation pressures ranged from 550 psi to 16,938 psi with an average pressure of 4,103 psi.
Fig. 7 shows the share of oil and gas reserves divided into 17 intervals of 1,000 psi ranging from zero psi to 16,000 psi.
Pressure intervals from 2,000 psi to 7,999 psi contained 82% of the oil reserves. Pressure intervals lower than 2,000 psi contained 2% of the oil reserves, and intervals above 8,000 psi had 15% of the oil reserves.
Gas reserves from 2,000 psi to 7,999 psi had 72% of the gas reserves. Pressure intervals below 2,000 psi contained 10% of the gas reserves, and pressure intervals above 8,000 psi had 18% of the gas reserves.
Oil, gas gravity
Oil gravity measurements for oil reserve data that have been released ran from 18° gravity to 61.8° gravity and averaged 34° gravity.
Fig. 8 shows the share of oil reserves separated into 10 intervals of 5° gravity from 15° to 60° and the share of gas reserves in 11 gas specific gravity intervals of 0.02 from 0.54 to 0.86.
Eighty-nine percent of the oil reserves were in gravity intervals of 30° to 35°. Oil gravity intervals lower than 30° had 8% of the reserves, and oil gravity intervals 40° and higher had 3% of the oil reserves.
With gas reserves, specific gravity was used instead of API gravity measurements with water having a specific gravity of 1.0. Specific gravity measurements ranged from 0.54 to 0.86 and averaged 0.60.
These were divided into 11 intervals of 2° each. Ninety percent of the gas reserves were from 0.58 to 0.65 sp gr intervals. Sixty percent of the gas reserves had a specific gravity of 0.60, and 12% of the gas reserves had a specific gravity of 0.62.
Specific gravity measurements lower than 0.58 had 6% of the gas reserves, and specific gravity measurements 0.66 or higher had 3% of the gas reserves.
Water saturation
Published measurements of water saturation with percent of reserve data reported ranged from 5% to 80% with average saturation of 29%.
Fig. 9 shows the share of oil and gas reserves divided into intervals of 5% water saturation from 5% to 80%.
Ninety-one percent of oil reserves were in four intervals of water saturation from 15% to 30%. Less than 1% of the oil reserves had a water saturation below 15%, and 7% of the reserves had 35% and above water saturation.
Eighty percent of the gas reserves were in the interval between 15% to 30% water saturation. One percent of the reserves were below 15% water saturation, and 19% of the reserves were above 35% water saturation.
Water depth
Depth of water in which drilling was done tells nothing about the amount of reserves but may help indicate the locations of producing reservoirs by age.
Review of published information for production is from less than 3,350 ft of water (Fig. 10). Oil reserves ranged from a minimum depth of 3 ft to a maximum depth of 3,318 ft with an average depth of 164 ft. Gas reserves ranged from 9 ft to 3,318 ft of water and averaged 158 ft of water.
Water depths for Pleistocene producing formations ranged from 12 ft to 2,858 ft and averaged 260 ft. Water depths for Pliocene formations ranged from 12 ft to 2,884 ft and averaged 161 ft. Water depths for Miocene formations ranged from 3 ft to 3,318 ft and averaged 165 ft. Water depths for Oligocene formations ranged from 16 ft to 66 ft and averaged 33 ft. Water depths for Jurassic formations ranged from 16 ft to 60 ft and averaged 37 ft.
Different property values may help subsurface exploration outline locations where the best reserves could be expected. If the water depth is less than 3,300 ft, the following suggestions may be made.
The largest share of reserves by reservoir depths for oil and gas was between 6,000 ft and 13,000 ft.
Largest share of reserves by oil pay thickness was from 40 ft to 140 ft. Largest share of reserves by gas pay thickness was above 20 ft and below 140 ft. Largest share of reserves by acreage for oil and gas reserves was between 5 acres and 4,000 acres.
Largest share of reserves by porosity for oil and gas reserves was between 26% and 32% porosity. Largest share of reserves by formation pressure for oil and gas reserves was above 2,000 psi and below 8,000 psi.
Largest share of reserves by oil gravity was above 30° gravity and below 40° gravity. Largest share of gas reserves by specific gravity was above 0.58° and below 0.66°.
Largest share of reserves by water saturation for both oil and gas was from 15% to 30%.
Areas with property values outside of those mentioned above should be avoided until more data are released that require changes or corrections to these suggestions.
Bibliography
Bureau of Economic Geology, “Atlas of Northern Gulf of Mexico gas and oil reservoirs,” The University of Texas, Austin, 1999.
Minerals Management Service, “Estimated Oil and Gas Reserves, Gulf of Mexico, Report 2002-07,” Dec. 31, 1999.
Haeberle, F.R., “Gulf of Mexico reservoir properties are helpful parameters for explorers,” OGJ, June 27, 2005, pp. 34-37.
Haeberle, F.R., “Gulf hydrocarbon trap types provide exploration insights,” OGJ, Dec. 12, 2005, pp. 36-42.
The author
F.R. Haeberle ([email protected]) is a consulting geologist who moved to Delaware, Ohio, after 20 years in Dallas. He has worked for Standard Oil Co. of Texas, Atlantic Refining Co., Mobil Oil Corp., Mayfair Minerals, and J.J. Lynn Oil Division. He holds BS and MS degrees in geology from Yale University and an MBA degree in finance from Columbia University Graduate Business School.
