Gulf of Mexico (GOM) OCS Assessment
by Jean Laherrere

Our methodology used for the assessment of several reports ("Undiscovered Petroleum Potential" 1994; The world's gas potential" 1996, "The world's non-conventional oil and gas" 1998) and published in several articles (see references), is based on five important points:

-1/ selection of a natural domain, namely the Petroleum System or a very large area (continent or the world);

-2/ use of proved+probable=P50 reserves which has only a small field growth (if any);

-3/field distribution on a size-rang log-log display being close to a parabolic fractal (see Laherrere 1996 & 1998);

-4/ creaming curve being the cumulative number of fields per category versus the cumulative number of New Field Wildcats modeled with a hyperbola (or several). The interactive combination of the two models 3/ &4/ gives the ultimate distribution first the yet-in-the ground and second, after applying constraints, the yet-to-find;

-5/oil equivalence of 1 boe=10 kcf in order to be close to the price equivalence, the gas-to-liquid conversion ratio and the volume in place as gas is recovered about twice more than oil from the volume in place and the natural distribution is the volume in place and not the reserves depending on economic and technical considerations.

The GOM OCS is only a part of a (or several) Petroleum System (s) which extends into the shallower waters, but it is a homogeneous base of data with a large number of fields which is worth to apply our modeling. But MMS uses proved values, leading to field growth (or reserve appreciation). We will consider first the method without any field growth and later how to introduce field growth. MMS use a calorific equivalence of 1boe=5.6kcf, giving an undue advantage to gas in the assessment of oil+gas.

-1- Without any field growth

From the 920 fields of the 1996 MMS report found on the web ranked by decreasing order, the display size-rank on a log-log format was plotted for each decade 1959,1969, 1979, 1989 and 1996 for oil+gas; and later for oil only and then for gas only. Each plot is curved and it is obvious that the plot stays about the same for the largest fields. We extrapolate the ultimate plot as a parabola fitted to the largest fields. As there is a range for drawing the parabola, we need another independent approach to better draw this parabola. We analyze the cumulative number of fields per category of a certain size (for oil >200 Mb, >100 Mb, >50 Mb, >10 Mb, >1Mb) versus time and later versus the cumulative number of fields (proved+unproved) reflecting the exploratory activity (as we lack the cumulative number of New Field Wildcats). This display, called the creaming curve, is showing the typical diminishing return which is usually modeled with an hyperbola as the simplest fit. The asymptote of the best fitted hyperbola for a category is compared with the parabola. Interactively the best fit between the parabolic fractal and the creaming curves is after trial and errors decided and reported in table 1

-Oil and gas

The 918 fields with reserves contained 39.8 Gboe. The cumulative discoveries versus time expressed in percentage of this total shows a quick return until 1975 where 80% were already found and a slow down after.

The parabolic fractal display shows that the first 15 largest fields were already found by 1969 and in 1989 the rank for over 100 Mboe was 119 when for 1996 it is only 120. The number of fields over 100 Mboe is best fitted to an hyperbola of an asymptote of 135 and the best parabola (S1= 750, a=-0.01, b=0.2) gives a number of 135 fields over 100 Mboe. The global best fit (rounded fit) is as follows for the number of fields per category:

category 1996 parabola hyperbola

>200 Mboe 44 52 50

>100 Mboe 120 131 135

>50 Mboe 202 284 270

The fit parabolic fractal (PF) is used to compute the number for the category below (>10 and >1 Mboe) the difference between the PF and the present data is the yet-in-the-ground. But it is obvious that there are asymptotic values, needing an infinite amount of exploratory activity and time. In a mature basin it is difficult to envisage that the ultimate amount of fields (or of NFW) will be more than twice the present number (1000) and we take a maximum of 2500 fields. In the creaming curve the value for 2500 fields is 48 fields>200 Mboe, 129 fields>100 Mboe and 246 fields>50 Mboe. In table 1 we take the yet-in-the-ground number per size (>200, 100-200, 50-100, 10-50, 1-10 Mboe) and reduce the number, a few for the large fields and many for the small fields to obtain a number yet-to-find which seems reasonable. We have obtained 51, 130 and 262 for the first three category and 818 for >10 Mboe and around 1600 for >1 Mboe. The corresponding reserves are given in table 1 as 50 Gboe for >10 Mboe and 53 Gboe for >1 Mboe. The economic threshold could be applied. We assume that it is around 1 Mboe as many fields of this size have been developed. The 53 Gboe is not far from the 51 Gboe from the creaming curve for 2500 fields but very far from the 75 Gboe of the ultimate in the ground.

If the total potential is around 52 Gb (average between 51 and 53 Gb) and the present total discovered is 39 Gboe. The total undiscovered is around 13 Gboe, quite apart from the 25 Gboe of report 96-47 for the mean undiscovered recoverable resources and from the range mini-maxi 21-30 Gboe.


From the 653 oilfields (600 developed with 92% of the reserves) totaling 12.8 Gb, after several trial and errors we have drawn a PF (S1=540, a=0.08, b=0,23) and the hyperbolas for the creaming curves and we obtained the number per category:

category 1996 parabola hyperbola for 2500 fields

>200 Mb 11 16 14 13

>100 Mb 36 41 41 40

>50 Mb 75 89 88 82

With an ultimate of 17 Gb, the total yet-to-find >1 Mb is 4.5 Gb, again far from the 8.3 of MMS mean estimate of recoverable undiscovered and from the range mini-maxi 6-11 Gb.


The 920 fields with gas reserves totaling 151.9 Tcf we have drawn the following parabola (S1=3300, a=0.08, b=0.18) and hyperbolas:

category 1996 parabola hyperbola for 2500 fields

>1100 Gcf 21 27 25 25

>560 Gcf 66 72 75 72

>280 Gcf 145 168 170 162

The yet-in-the-ground >5 Gcf is 116 Tcf but the yet-to-find 44 Tcf (or 51=203-152 for the creaming curve at 2500 fields). It is far from the MMS mean estimate of 96 Tcf and from the range mini-maxi 82-110 Tcf.

-adding "oil" and "gas"

The sum of the two last assessments of oil and gas give a total of yet-in-the-ground and yet-to-find >1 Mboe of 31 Gboe and 12 Gboe against 36 and 13 Gboe for the combined first assessment. It is within 10% difference, which is for us similar.

-synthesis: table 1

Table 1 gives all the figures for a breakdown of certain sizes but the figures and the spreadsheet can give any other breakdown if the economic threshold is different:

-2 with field growth

From the web we have the field values reported as 1995 and as 1996. I have compared the two listings and found only that only 859 fields out of the 899 1995 fields are in the 1996 base (where is WC 565 1971 discovery with 802 Gcf?). Out of 859 fields there are 205 negative revisions (24%) for gas against 371 positive revisions, when with SPE/WPC 1997 definitions only 86 negative revisions are permitted, it means that here proved has a probability of only 75%. For the 616 oilfields, there are 154 negative revisions against 251 positive ones, again proved with P75. The global changes for these 859 fields was 4% for oil and 6% for gas, and 5% for oil+gas for one year growth: it is quite different from the MMS field growth model as an annual growth model is at 5%/a at around 10 years after discovery . But 50% of the GOM OCS was discovered already 30 years ago. For the last 10 years only 6% of the total discovered was found.. Applying the MMS field cumulative growth model to the 40 Gboe, 12.7 Gb and 152 Tcf already found, gives, after growth, an increase of 6.5 Gboe, 2 Gb and 36 Tcf in the next twenty years, which is close to MMS reserve appreciation of 8 Gboe, 2.2 and 33 Tcf of 96-47. Applying the annual growth model, the increase is only less than 1.5% and 5% as from this queer increase from 1995 to 1996.The change from 1995 to 1996 seems to be a correction of a first attempt which was not very good, explaining why 20 fields reported in 1995 have disappeared in 1996. and also why the change versus discovery year look erratic and not increasing for the last discoveries. The trendline of the changes is constant from 1947 to 1996 around 5%.

The use of proved reserves obliges to neglect probable and possible reserves and leads to field growth or reserves appreciation. The use of proved+probable as it is done in North Sea (as it is permitted by the UK accounting procedures) conducts to a global small reserves appreciation

The MMS field growth model is applied to the fields larger than 50 Mboe and to the creaming curve for reserves and for number of fields over 100 Mboe. We plotted the parabola with the best fit with the creaming curves and found the following number of fields:

category 1996 parabola hyperbola for 2500 fields

O+G >100 Mboe 120 145 158 148

O >100 Mb 36 45 48 45

G >560 Gcf 66 79 90 84

The parabolas for oil+gas, oil and gas after field growth (AFG) are slightly different from the parabolas before field growth. In fact the distribution for oil AFG is more harmonious than BFG. The distribution oil+gas is bumpier than the data for oil and for gas, showing that the oil equivalence ratio is not a good one.


In the drawing of the PF before field growth, the ultimate distribution is supposed to take care both of the undiscovered and of the future revisions if the change is not to great as it is when dealing with proved+probable.

The parabolic fractal AFG as drawn includes both the future revised fields and the undiscovered. The difference between the PF AFG and the present data is constrained by the activity assumed maximum order to give the yet-to-add which includes the reserves appreciation.

The undiscovered defined as the yet to add outside the reserve appreciation of the discovered is as follows:

now BFG yet to find BFG yet to add AFG res apprec undiscovered

oil+gas Gboe 40 13 18 6.5 12

oil Gb 12.7 4.5 5.7 2 4

gas Tcf 152 45 48 36 12

The discrepancy between the separate estimate of oil and gas compared to the combined estimate of oil+gas indicates the uncertainty of such assessment.

In fact the undiscovered after field growth is less than the undiscovered estimated before field growth as in this case the field growth is included for a large part in the undiscovered and have to be excluded.


The present data on GOM OCS need to be improved as the last revision of 1996 versus the first estimate of 1995 shows too much change to be attributed to only pure field growth.

An interactive combination of modeling of parabolic fractal and creaming curves allows to draw the ultimate distribution of the basin and to extract with the necessary constraints the recoverable yet to find.

The use of proved is an handicap in the methodology and it is recommended to try to use proved+probable value for such assessment.

Our assessment of the recoverable ultimate is about 18 Gb for oil, 210 Tcf for gas and 56 Gboe for oil+gas, very different from MMS estimate of 23 Gb for oil, 275 Tcf for gas and 72 Gboe for oil+gas.

Our estimate is a first look and could be increased with no much problem if it is felt that the data is unreliable and that there are more plays still to be discovered.

Without knowing the reliability of the data and without better knowledge of the geology and the seismic of the GOM, I feel unable to estimate the range of uncertainty. But I feel that the MMS range is too narrow: 21-26 Gb for oil should be 17-26 Gb. 262-289 Tcf should be 200-290 Tcf for gas.

MMS has done a very interesting work in releasing most of the data and many reports. MMS assessment could be improved with a more accessible method where every steps could be displayed.

I want to thanks MMS for receiving me and accepting to discuss their methods and to compare with mine. I am ready to help MMS in improving assessment methods which are still imperfect. But the data have first to be improved, mainly by including probable reserves ot a probabilistic estimate of the discovered, as the undiscovered.

References: JHL= J.H.Laherrere

-JHL, D.Sornette 1998 " Stretched exponential distributions in nature and economy: fat tails» with characteristic scales" European Physical Journal B 2, April II, p525-539 :


or loading (1 Mo):

-Campbell C.J, JHL 1998 "The end of cheap oil" Scientific American March p80-85

see: or

-Perrodon A., JHL, C.J.Campbell 1998 "The world’s non-conventional oil and gas" Petroleum Economist March report 113p see

-JHL 1997 "Future sources of crude oil supply and quality considerations" DRI/McGraw-Hill/IFP conference "Oil markets over the next two decades: surplus or shortage?" June 12, , 10p- 20 fig

-JHL, A.Perrodon, C.J.Campbell 1996 "The world’s gas potential" Petroconsultants report July, 200p, CD-ROM

-JHL 1996 "Distributions de type fractal parabolique dans la Nature"-Comptes Rendus de l’Académie des Sciences- T.322 -Série IIa n°7-4 Avril p535-541

-B.Popescu, JHL 1995 "Undiscovered potential of five main Romanian Petroleum Systems" Petroleum Geoscience - vol 1 n°4, Nov- Appendix, p347-349

-JHL, A.Perrodon, G.Demaison 1994 "Undiscovered Petroleum Potential" Petroconsultants report - 383p - March


updated 2000 March 1