ABSTRACT

In the Tertiary-Pleistocene clastic sediments, compaction disequilibrium is the primary cause of geopressure compartmentalization in the deep water of the Gulf of Mexico. A sealed compartment is the key for exploration success in any play concept. Estimating the shift of pore pressure envelopes in relation to geopressure compartmentalization is essential in predicting objectives risk.

The actual pressure measurements (i.e., RFT's and MDT's) in reservoir type sections show a cascade profile with depth in the geopressured (abnormal) column. The amount of shift in the pressure envelopes designates the sealing and retention capacities. A correlation was found between the progressive-regressive shift and exploration success in some of these deep water wildcats.

Substantial hydrocarbon accumulations were found where a considerable progressive shift was recorded (> 1000 psi), i.e., Green Canyon 809 #1, 72 # A-1, 506 #1, Mississippi Canyon 211# 1, and Viosca Knoll 1001#1. Marginal reserves or wet reservoirs were found where a small shift (< 1000 psi) was noticed, i.e., Garden Banks 543 #1, 142 #1; Green Canyon 908#2; and Keathley Canyon 255#1.

On the other hand, where pressure envelopes show regressive behavior, the target reservoirs were unsuccessful, i.e., Garden Banks 248 #1 and Viosca Knolls 912#1.

Bore-hole trajectory in relation to the geopressure profile of each objective should be investigated before making a conclusive risk assessment. Study of this correlation on a basin to basin basis is imperative for the productive evaluation of seal integrity, compartments communication and potential hydrocarbon trapping.

Concepts and Methods

Pore pressure (PP) is the product of interaction between fluid and rock matrix due to compaction. Compaction is caused primarily by the weight of the deposits (overburden) coinciding with basin subsiding. An increase of principal stress with depth due to sedimentation load accelerates geopressure. Pressure progresses with depth in a cascade fashion (Fig. 1) in a tectonically relaxed system (Shaker, 2001). Seal failure and upward communication are the main cause of pressure regression (Fig. 2).

The measured actual pressure values, such as Repeat Formation Tester (RFT) and Modular Dynamics Tester (MDT), establish the progress and regress in the pressure envelopes. The sandwiching of seals between successive reservoir compartments (where PP is measured) is responsible for the very slow pressure decay in the entire subsurface section. Principal stress, pressure decay rate, and structural and stratigraphic settings are the driving mechanism behind progression and regression shifts in the subsurface.

Progression System

Shale beds negligible permeability and geological age lead to an immeasurable decay process in the progressive system. The PP decay in the progression system is usually very slow. This is due to the fact that the initial sealing capacity of the geopressure compartmentalizations is still intact and follows the compaction disequilibrium laws as long as sedimentation and subsiding are in progress. This leads to relatively large envelopes. The success of finding commercial hydrocarbon accumulation depends on the size of this pressure envelope and its immediacy to the fracture limit.

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Figure 1. Schematic geopressure profile shows pressure envelopes progression with depth. The PP in the reservoir follows a linear hydrostatic gradient in the reservoir. On the other hand, predicted PP in the seal has a higher exponential gradient. The difference between pressure envelopes represents progression shift.

Figure 2. Schematic geopressure profile explains the most likely reason for pressure envelopes regression in the Gulf of Mexico. Upward communication is represented by dashed pathway at the right of the figure. PPP represents predicted pore pressure, and MPP represents measured pore pressure.

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Regression System

PP regression exists in cases where seals fail and upward communication through faults, salt walls, and facies interface takes place (Fig. 3). The PP decay in the regression system is relatively high due to the presence of large negative pressure difference between the seal and the underlying compartment (Fig. 4). The measured pore pressure in deeper reservoirs retreats to the same pore pressure gradient envelope as that of the shallow ones.

Figure 3. A conceptional geological cross section on a salt ramp explains PP progression and regression. Progression takes place where the system is sealed. Regression exists where communication between the deep reservoirs and shallow ones takes place.

Figure 4. Schematic geopressure profile exhibits the pressure decay process in a regressed system over time throughout the seal.

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Gulf of Mexico Cases

Hydrocarbon accumulation in the subsurface of geopressured structural and stratigraphic traps relies mainly on the seal integrity. The shift in the geopressure envelopes indicates the capability of the seal (cap) to exert a certain pressure inside the compartment. Hydrocarbon presence in the reservoir inflates the virgin pressure in the reservoir beds. The introduced pressure, due to the presence of oil and gas, is proportional to the density and height of the hydrocarbon column. A direct relationship was found between the shift direction, as progressive or regressive, and the presence of oil and gas in the objective targets. Moreover, the size of the progressive shift impacts the size of oil and gas accumulations.

Large Progressive Shift

Most of the commercially discovered fields in the deep water of the Gulf of Mexico are characterized by large progressive geopressure shifts. This shift takes place at the interface between the cap shale seal and the targeted reservoir compartment. Popeye (Green Canyon 72 # A-1), Fuji (Green Canyon 506 #1), Mickey (Mississippi Canyon 211#1), Ursa (Mississippi Canyon 809 # 1) and South Rampowell (Viosca Knoll 1001#1) fields show shifts usually exceeding 1000 psi (Fig. 5).

Figure 5. Pressure-depth (P-D) plot of Mississippi Canyon Block 809 Well#1 shows the large progressive pressure envelopes coinciding with the presence of oil and gas in Ursa field. The GOM line on the left graph represents the regional Gulf of Mexico hydrostatic gradient. Pressure progressions are represented by the red arrows. Oil and gas pressure gradients are shown as green dashed and red dotted lines, respectively. Notice gas reservoir needs larger sealing capacity (progressive shift) than oil.

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Small Progressive Shift

Most of the economically marginal finds and unsuccessful wildcats are characterized by a small progressive shift (<1000 psi). Pressure decay rate, fracture gradient acceleration, and structural failure contribute to the weak progression shift. The plugged and abandoned wildcats drilled in Garden Banks 543 #1, Garden Banks 142 #1, and Green Canyon 908 #2; and Keathley Canyon 255#1 support this relationship (Fig. 6).

Regression

Pore pressure regression in the subsurface can be caused by several structural settings and stratigraphic events. Faults and salt walls act as fluid conduits from highly pressured deep compartments to lower pressured shallow ones. This leads to a pressure regression in the lower reservoirs (compartments). Consequently, in the deep compartment the pore pressure retreats to an envelope equal to the pressure in shallow reservoirs.

The basal part of the Garden Banks 248#1 (Fig. 7) and Mississippi Canyon 211 #1 shows this pressure regression phenomenon. In this case the target objectives were barren of hydrocarbons. Titan Prospect (GB 785 # 1) tested a measured PP in the target reservoir of 1.8 ppgMWE less than the predicted surrounding shale (12.4 vs.14.2 ppgMWE) at depth 18500 ft. The well was plugged and abandoned (Dry Hole Seminar, 2000).

Figure 6. P-D plot of Garden Banks 543 #1 shows the small progressive shifts (red arrows). Note the section between 11000 ft and 15000 ft has the same pressure envelope. This suggests that the stratigraphic column at this interval is in communication and trapping integrity is of high risk. The well was plugged and abandoned.

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Conclusions

Integrating the geopressure compartmentalization data to the prospect geological model is essential for trapping and risk assessments. Sealed compartments usually are represented by a progressive geopressure shift. The size of this shift reflects the capability of the cap rock to exert the introduced pressure by emplacing hydrocarbon in the reservoir. On the other hand, pressure regression indicates a seal failure and the incapability of the reservoirs to retain oil and gas. Therefore, bore-hole trajectory needs to be assessed in relation to compartmentalization in the prospective basin in order to evaluate trap integrity in each exploration target.