DEEP PORE PRESSURE PREDICTION IN CHALLENGING AREAS, MALAY BASIN, SE ASIA

Transcription

1 T: E: W: Pore Pressure Prediction in challenging areas - IPA Publication May 2011 PROCEEDINGS, INDONESIA PETROLEUM ASSOCIATION Thirty-Fifth Annual Convention & Exhibition, May 2011 IPA11-G-022 DEEP PORE PRESSURE PREDICTION IN CHALLENGING AREAS, MALAY BASIN, SE ASIA Stephen O Connor* Richard Swarbrick* Jamaal Hoesni** Richard Lahann*** ABSTRACT Rapid burial and high rates of sedimentation in the Malay Basin has lead to development of overpressure by disequilibrium compaction. Overpressure developed by this process can be quantifi ed using industry-standard techniques that rely on porosity/effective stress relationships. However, where thermally-driven secondary processes create overpressure, porosity-based analysis that uses sonic (or seismically-derived velocity data) and resistivity data as a measure of porosity change underestimate overpressure. These processes will be active in relatively shallowly-buried shales in basins with high geothermal gradient such as the Malay Basin. Using comparative datasets from several regions where secondary overpressure generation are present (Gulf of Mexico; Halten Terrace, Mid- Norway and the Malay Basin), we discuss such secondary mechanisms and their quantifi cation by integrating velocity vs. density cross-plots with understanding of basin history. Analysis of velocity vs. density relationships is a powerful tool to help * GeoPressure Technology Ltd. ** PETRONAS *** Indiana Geological Survey discriminate overpressure generating processes and using this technique and log data from the Malay Basin, we identify load transfer (where rock compressibility is affected) as present, in addition to unloading and cementation effects documented by previous authors. Overpressure generated by load transfer may only be partially detected by fl uid expansion-based relationships such as Bowers (1994), leading to inaccurate pre-drill pressure predictions. The identifi cation of load transfer (and cementation) processes and their quantifi cation is vital to accurate prediction of pore pressure in hydrocarbon-charged reservoirs in the Malay and other basins worldwide INTRODUCTION The Malay Basin is a Tertiary trans-tensional rift basin, offshore peninsular Malaysia. In this basin, both gas-rich and mixed oil/gas zones are present. Over 12km of fi ne-grained sediments were deposited in the last 35 Ma, with rates of sedimentation as high as 1000m/Ma are calculated for the syn-rift phase (Madon, 2007). The depth to the start of overpressure varies across the basin and is shallowest in the basin

2 centre, e.g. wells such as Dulang-1 and Tangga-1, km TVDss. The Middle Miocene Unit F shale acts as a regional seal (Madon, 2007). All well locations named in this study are shown in Figure 1. A similar observation was made in Singh and Ford (1982) based on analysis of over 150 exploration wells. The Middle Miocene unit E represents a pressure transition zone. On the basin fl anks, overpressure starts deeper, often at 3.0 km TVDss e.g. Larut-1. In the SW of the Malay Basin, in the vicinity of wells such as Beranang 6F-18.1 and Resak 6F-18.2, anomalously high overpressure occurs at depths of 2.6 km TVDss, sealed by the on-lapping, transgressive shale of unit L (Lower Miocene). Shale seals have a strong infl uence on overpressure distribution, as do rates of sedimentation and subsidence (Madon, 2007). To-date, current drilling has rarely penetrated these deeper, highly overpressured parts of the Malay Basin. Those wells that have drilled deep, such as the Bergading Deep and Sepat Deep-1 wells, encountered High Pressure/High Temperature (HP/HT) conditions, severe mud losses, well kicks and other operational diffi culties such as stuck pipes, hole stability and hole caving while drilling of the well (Mohamad et al., 2006). Prior to 1994, 80% of exploration and appraisal wells were terminated due to overpressure in the Malay Basin (Shariff, 1994). Careful and accurate pore pressure prediction, therefore, will be the key to defi ning the next exploration phase of the Malay Basin. A major component of this process will be the understanding of those processes that create overpressure, and the identifi cation of these processes in this basin. Disequilibrium compaction is believed to be the primary causal mechanism for the overpressure in the basin (Madon, 2007), however, due to the high geothermal gradient (51.8oC/km; Halim, 1994), we show evidence in this paper that secondary overpressure mechanisms are also important. Secondary mechanisms such as fl uid expansion and cementation have previously been identifi ed by Hoesni et al. (2007) in the Malay Basin. In this paper, we review this work and show evidence of additional processes, related to changes in rock compressibility (load transfer). Conventional porosity-based pore pressure analysis using sonic/seismic velocity and resistivity data as a measure of porosity retention, underestimates the overpressure effect of these secondary overpressure mechanisms. Using velocity and density data from a well in the South and North Malay Basin, Wells A and B (Figure 1), we illustrate the methodology to identify overpressure generation mechanisms in the Malay Basin by using velocity vs. density cross-plotting, and discuss implications for pre-drill prediction. We also review approaches to allow for these mechanisms i.e. often empirical fi ts to local data, and results from our work in other basins, where we have attempted to quantify these mechanisms by integrating basin history with rock properties and temperature data. MECHANISMS OF OVERPRESSURE GENERATION In environments such as the Malay Basin, Gulf of Mexico and Nile Delta, where rates of sedimentation are high, the sediments are young and with low geothermal gradients, pore pressure profi les through shale-dominated sequences can be estimated confi dently using seismic velocity, wireline sonic and resisitivity data. Assumptions are that the basin is extensional and that the main mechanism of overpressure generation is under-

3 compaction as a result of ineffective dewatering, referred to as disequilibrium compaction (Hubbert and Ruby, 1959). These profi les are typically overburden parallel (Swarbrick et al., 2002), i.e. increasing pressure with increasing depth with (a) constant porosity, implying (b) constant vertical effective stress (overburden minus pore pressure). Such pore pressure profi les are present in the North Malay Basin (Yussof and Swarbrick, 1994; Figure 2). Another example is shown in Figure 2 in Madon (2007). In both these examples, a series of thin and/or undrained, thicker sands encased in shales contain WFT (Wireline Formation Test) data, that acts as a proxy for measuring shale pressures. In both cases, sharp pressure transition zones are present involving the deep reservoirs, producing overburden-convergent pressure profi les, and suggesting secondary mechanisms are likely present (Figure 2). As porosity is reduced to low values as a result of mechanical compaction during burial and the temperature increases (for instance, above 70oC in some basins; Gulf of Mexico, Bruce 1984), mineralogical changes occur in the shales. These changes lead to the two main processes of secondary overpressure generation, fl uid expansion/volume change and load transfer (or framework weakening). Fluid expansion/volume processes include dehydration reactions such as gypsum to anhydrite, and smectite to a more dehydrated form. Smectite to illite transformation produces water released and silica, which will tend to precipitate locally. The release of bound water into sediment pores is minor in terms of generating overpressure (Swarbrick, and Osborne 1998). Maturation of hydrocarbons, particularly in the case of gas generation, produces rapid volume expansion, reducing effective stress and increasing pore pressure. These fl uid expansion mechanisms such as gas generation and dehydration reactions generate overpressure that can be calculated using Bowers (1994), a relationship that is based on velocity and changes in effective stress, where porosities are low. Other mechanisms involve changes in rock compressibility. If the rock compressibility is increased, there will be an extra load superimposed onto the fl uid phase as a result of the applied stress. These processes involve weakening of the framework and pore collapse driven by clay diagenesis and dissolution of framework-supporting grains such as kerogen and K-feldspar (Lahann (2001, 2002) and referred to as load transfer in Swarbrick et al, 2002). These reactions only occur where the temperature exceeds about 80oC, although in younger sediments the temperature at onset is more typically > oC. Pressures generated in shales, where porosity is low, temperatures are increasing, and clay mineral diagenesis and hydrocarbon generation are ongoing, are transmitted to any associated sands, particularly sands of restricted extent such as turbidites. Many pressure depth profi les in shale-dominated facies e.g. Nile Delta, Mann and MacKenzie (1990) show increasing overpressure with depth, proof of reservoir isolation and close coupling with the enclosing shales e.g. in the Malay Basin, Figure 2. Thicker, sands that are laterally extensive have more capacity to allow the dissipation of these pressures if a leak or exit point is established via continuous reservoir or fault networks to shallower levels. Examples of reservoirs that have less pressure than the surrounding shales and are laterally draining pressures (and fl uids) include the Paleocene fans of the Central North Sea (Dennis et al., 2000, 2005), as well as the Egga Sandstone Formation,

4 Ormen Lange fi eld reservoir, Mid-Norway, and Cauvery Basin, East India reported in O Connor and Swarbrick, (2008). METHODS TO IDENTIFY SECONDARY PROCESSES OF OVERPRESSURE GENERATION Commonly used methods to estimate the magnitude of pore pressures, such as Equivalent Depth or Vertical and Eaton Ratio or Horizontal (Eaton, 1975) are based on the detection of anomalously high porosity for depth of burial. The porosity is high due to the ineffective dewatering of shales during burial, whereby part of the increasing vertical load of the overburden is transferred to the fl uid phase, increasing pore pressure above hydrostatic. Both relationships are derived from Terzaghi (1953) (Equation 1) based on soil mechanics and related to the values of log data such as sonic and resistivity compared with those values associated with porosity-loss on a normal or primary compaction curve. Where, S v = σ v + P f (1) σv = vertical effective stress P f = pore pressure S v = vertical stress, derived from density data or sonic-derived density data from the equation; As mentioned above, processes such as gas generation or load transfer increase the pore overpressures reducing the grain-to-grain contact stresses (effective stresses). However, compaction is mostly irreversible, therefore porosity-based pore-pressure-prediction methods (e.g. Equivalent Depth Method) will not detect these increases in pressure as no associated porosity anomaly is present, tending to underestimate pore pressures caused by mechanisms other than disequilibrium compaction. Velocity/density vs. cross-plots can be used to identify the presence of overpressure generated by these other mechanisms see Figure 3. In the case of gas generation, reduction of effective stress, has the effect of reducing density a very small amount (elastic rebound) but has a much greater impact on the velocity. Hence the steep downwards trend associated with unloading (Figure 3; modifi ed from Hoesni, 2004). Bowers (1994) has developed this approach to distinguish between disequilibrium compaction and other overpressure generating mechanisms. An example of a typical resulting profi le for unloading is also displayed in Chopra and Huffman (2006). Normal compaction and disequilibrium/under-compaction display typical increasing velocity and density magnitudes. This profi le or primary compaction curve initially shows increase in density with relatively little velocity response, a pattern which corresponds to mechanical compaction with little/no cementation, so that grain-grain contacts are minimal and velocity increases slowly. With increasing effective stress and as compaction proceeds more graingrain contacts are made, giving a distinctive curvature of the profi le with depth where velocity increases more rapidly relative to density (Gardner or Bowers relationships for shales in Bowers, 2001). Where load transfer occurs, the transformation of framework-supporting grains to hydrocarbons (oil and/or gas) in the case of kerogen, smectite to illite transformation and/ or porosity as K-feldspar dissolves causes an increase in density - if the system can allow some to escape. A decrease in velocity occurs as effective stress decreases.

5 Cementation such by silica will strengthen the rock framework, reducing permeability (aiding overpressure retention) and increasing the velocity of the shales. Density increases may be variable, depending on the types of cement and its distribution. Log data is the primary input for these cross-plots, therefore, logs were initially processed, depth matched and shale data extracted using a cutoff based on gamma-ray data as an indicator of lithology. Caliper logs were used to remove the effects of wash-out that affect borehole integrity and can cause inaccurate log responses. Finally, a moving average fi lter can be applied to remove spurious values such as high velocity spikes due the localised cementation. RESULTS FROM THE MALAY BASIN Evidence for secondary processes in the Malay Basin is presented in Figures 4 and 5. Both density and sonic data were available for these wells. In Well A, in the South of the Malay Basin, the top of overpressure is at 1.2km, whereby shale pressures increase parallel to the overburden (as predicted successfully by the Equivalent Depth Method, not shown). Below 2.0 km TVDss, shale pressures under-estimate the K and L reservoir pressures by psi. These reservoirs are un-drained, massive sands, overlain by thick shale sequences (Hoesni, 2004). The estimated pore pressure at TD is 7093 psi (15.8 ppg) (Hoesni, 2004). Figure 4 displays the velocity/density relationship for this well. Deviation is observed from the primary compaction curve of Bowers (2001) at temperatures of 120oC, although more signifi cantly at 160oC. This signature of increasing velocity and density is identifi ed by Hoesni et al. (2007) as suggestive of chemical compaction/ cementation effects. In Well B, sandy lithologies pre-dominate deeper than in Well A, therefore overpressure commences at 1.8 km TVDss. Shales dominate below this depth, and reservoirs display increasing overpressures. High mud-weights were used to control the pore pressure in this well, and indeed, shale pressure prediction using the Equivalent Depth Method (not shown) suggest mud-weights used in the well were signifi cantly below the shale pressures. The well was abandoned, due to wellbore instability problems attributed to high pore pressures; at 2747m TVDss, mud-weights used were 17.6 ppg (Hoesni, 2004). Figure 5 displays velocity and density for Well B as well as the estimated borehole temperatures. The depth at which the shale pressure interpretation by the Equivalent Depth Method proves to be inaccurate i.e. under-estimates pore pressures as confi rmed by WFT measurements in thin, encased sands, is approximately 2.4 km TVDss (Hoesni, 2004), corresponding to 124oC using a geothermal gradient of 51.8oC/km (Malay Basin; Halim, 1994). A defl ection to slower velocity and increased density occurs at this temperature this trend is similar to that shown in Figure 3. The trend is representative of load transfer i.e. the transformation of kerogen to hydrocarbons (oil and/or gas), porosity as K-feldspar dissolves and/ or smectite to illite all of these processes affect rock compressibility and load the fl uid phase. DISCUSSION Madon (2007) states that disequilibrium compaction is the primary source of overpressure generation in the Malay Basin centre, caused by high sedimentation rates. Modeling results suggest that overpressure generated early during the synrift phase when sedimentation rates were high (>1000 m Ma). As post-rift rates were lower (<

6 500 m Ma), no overpressure was generated in this phase, such that current overpressure patterns are due to re-distribution of overpressure via faults and regional seals (F and L shales). Evidence from our analysis suggests that additional overpressure mechanisms may exist in the Malay Basin, mechanisms that are not associated with a porosity anomaly and therefore problematical to detect. These mechanisms are identifi ed to be due to load transfer processes. Hoesni et al (2007) also provide evidence for secondary processes in the Malay Basin via unloading, although typical trends of rapid velocity loss (Figure 3) are not visible on many velocity vs. density cross-plots due to the effects of chemical compaction (via cementation) as typifi ed in Figure 4 from Well A. Hoesni et al. (2007) defi ne a model for chemical compaction where shale framework collapse with partial dewatering, followed by sequential fi lling of pore spaces in shales by cement occurs, in both storage (inter-granular) and connecting pores. This cementation results in enhanced seal capacity for shales acting as vertical barriers to migration. Analysis of velocity vs. density cross-plot data from Well B produces deviation from typical shale trends characteristic of normal compaction/disequilibrium compaction (i.e. a primary compaction curve) (Bowers, 2001). The defl ection to higher density and lower velocity is characteristic of load transfer, where rock compressibility is affected, resulting in a different compaction profi le. This type of signature is reported from the Gulf of Mexico by Lahann (2001, 2002), and associated with changes in rock compressibility by smectite to illite transformation during clay diagenesis at 80oC (Figure 6). However, several factors such as time and clay type affect the temperature of onset of this transformation. Data in Lahann (2002) from the Gulf of Mexico suggests that overpressures of psi can be attributed to this process. Typically, oC is the temperature range at which signifi cant overpressure can be generated by this method. The departure from the primary compaction trend in Figure 5 occurs at 120oC It is not possible without further analyses e.g. of shale samples to ascertain compaction and mineralogical state, to determine which process affecting rock compressibility is present/occurring. Unloading as defi ned by Bowers (1994, 2001), causes reduction in effective stress and velocity, assuming plastic and elastic sediment behaviour. Where processes occur that affect compressibility, this behaviour will be inelastic, and the compaction state of the rock permanently altered (Katahara, 2006). Although the velocity/effective stress model in Lahann (2002) for the Pathfi nder well resembles an unloading curve as discussed in Bowers (1994), this method may only offer a partial solution to pre-drill pore pressure prediction, where load transfer processes are present, such as the Malay Basin. In order to model the effects of changes in rock compressibility, a post-unloading compaction model as discussed in Lahann (2001, 2002) could be defi ned (if suffi cient data exists). This model can be applied to data which are too deep to be accurately modelled by a Bowersstyle unloading curve. The entire well profi le can be modelled with a primary curve, a Bowersstyle unloading curve, and a deep compaction model. Alternatively, the unloading interval may be interpreted by a mixing function that changes with depth from the primary model to the deep compaction model. This study (and Hoesni et al., 2007), would propose that there is also evidence for additional processes that generate overpressure in this basin, caused by the high geothermal gradients. An important outcome of this study is, therefore,

7 that more complete understanding of the effects and changes in rock compressibility are needed as these parameters cannot be measured/ predicted pre-drill. Pressure models based on sonic log or seismic velocity analysis data will not be accurate if the techniques mentioned in this paper are used e.g. Equivalent Depth, Eaton (1975) and potentially Bowers (1994). Clearly, in basins where temperatures are elevated e.g. the Malay Basin, velocity data could prove problematic below 2.0 km, therefore using seismic data will signifi cantly under-estimate pressures by 1000 s psi a major drilling safety issue. Potential solutions often rely on fi nding empirical fi ts to existing data and applying locally, or using relationships derived in different basins, and applied worldwide. An example of the former is cited in published analyses by Dolson et al. (2005) from the Nile Delta. In these datasets, seismic interval velocities are considered too fast for the Miocene section, resulting in inaccurate calculation of pore pressures. Using an Eaton exponent of 5.0 in Wells such as Akhen-1, however, provides a match with reservoir data. Geothermal gradient data of 25oC/km in Manzoni et al (1998) suggests Miocene shales of the Qantara Formation are likely affected by thermal processes, transmitted to these reservoirs. A more robust approach is to use velocity vs. density cross-plots in conjunction with knowledge of overpressure mechanisms, rock properties and understanding of basin history. For example, the Lower Cretaceous deep-water Lange Formation shales provide a continuous cover of fi ne-grained sediment over the Halten Terrace, Mid-Norway. From analysis of temperature data, the 100oC isotherm is generally shallower than the Lange shales and therefore the shales are in the window where these thermally-driven mechanisms could be a factor. Density log data increases with depth, indicative of a primary compaction curve. The suggestion is that overpressuring to the current levels proceeded independently of porosity loss i.e. there is a signifi cant component of secondary overpressure is the Halten Terrace region that post-dated compaction and has no associated porosity anomaly (Hermanrud et al., 1998). A burial curve based on composite log data for Well 6506/11-6 (not shown) demonstrate two periods of rapid burial (1) Turonian/ Campanian and (2) Plio-Pleistocene. Work by Skar et al. (1999) suggests that pressures were hydrostatic prior to this latest burial due to pressure bleed-off during the Tertiary hiatus. The relative contributions of the rapid loading during the Plio-Pleistocene (1.7 km of sediment; Norgård Bolås et al., 2005) and secondary contribution to the current Lange pore pressure profi le in Well 6406/2-3 are illustrated in Figure 7 (GPT/IHS, 2007). The blue line on the left is the hydrostatic (normal) pressure starting point at 3 Ma ago. The darker blue line to its right represents the pore pressure profi le after the rapid burial event, with a constant contribution to overpressure from rapid loading via ineffective dewatering. The current Lange pore pressure profi le (as defi ned by WFT data in encased intra-formational sands, often four per well, and defi ning a regional shale gradient) is indicated by purple line on the right of the fi gure. For the deeply buried rocks such as the Lange shales, already having low permeability, this additional overburden will be translated into overpressure (assuming no signifi cant dewatering) by: 1.7 km sediment thickness x (lithostatic gradient of 3.28 psi/m - water gradient of 1.45 psi/m) = 2711 psi

8 Therefore, at 4.0 km depth in Figure 7, using the purple line representing current pressures in the Lange indicates approximately 4350 psi of overpressure (1639 psi greater than recent loading history calculated above could have generated). In such old and hot rocks, seismic-based velocity prediction of pore pressure would be unreliable, as there is no porosity/effective stress link. The difference between dark blue and purple lines represents our best estimate of the contribution to overpressure from diagenetic changes in the shales. Velocity vs. density cross-plot analysis suggests load transfer (smectite to illite transformation, K-feldspar dissolution etc.) processes are active in these shales, with a reduction in velocity, and increase in density. CONCLUSIONS In conclusion, in the Malay Basin, there is a strong correlation between rate of sedimentation and overpressure development by disequilibrium compaction. However, as wells are drilled deeper into this Basin (and other basins world-wide below the 100oC isotherm), thermal processes in shales will result in secondary overpressure generation, and, if this overpressure is transmitted to reservoirs, pre-drill predictions of pore pressure will be inaccurate, compromising safety. Using traditional techniques of pore pressure prediction such as Equivalent Depth and Eaton (1975) will be inadequate. Bowers (1994) offers only a partial solution. New relationships will need to be developed based on integrating an understanding of basin history, shale behaviour, clay mineral diagenesis, thermal behaviour and geological time to successfully predict pore pressures in this, and other, hot and deep basins world-wide. ACKNOWLEDGMENTS The authors would like to extend their thanks to the organizing committee for the chance this work at the IPA Conference, 18-20th May, REFERENCES Bowers, G., Pore pressure estimation from velocity data: Accounting for overpressure mechanisms besides undercompaction. IADC/ SPE 27488, IADC/SPE Drilling Conference, p Bowers, G., Determining an appropriate pore-pressure estimation strategy. OTC 13042, Offshore Technology Conference. Bruce Smectite dehydration its relation to structural development and hydrocarbon accumulation in northern Gulf of Mexico Basin. AAPG Bulletin, v. 68, No.6, p Chopra, S and Huffman, A Velocity determination for pore-pressure prediction. The Leading Edge; December 2006; v. 25; no. 12; p Dennis, H, Baillie, J, Torleif, T, Holt, T and Wessel- Berg, D Hydrodynamic activity and tilted oil-water contacts in the North Sea. Norwegian Petroleum Society Special Publications Volume 9, 2000, Pages

9 Dennis, H, Bergmo, P and Holt, T Tilted oil water contacts: modeling the effects of aquifer heterogeneity Petroleum Geology Conference series 2005, v. 6, p Dolson, J.C, Boucher, P.J, Siok, J and Heppard, P.D 2005 Key challenges to realizing full potential in an emerging giant gas province: Nile Delta/ Mediterranean, deep-water, Egypt. In: Dore A.G and Vining, B (eds) Petroleum Geology: North-West Europe and Global Perspectives Proceedings of the 6th Petroleum Geology Conference, p Eaton, B The equation for geopressure prediction from well logs. Society of Petroleum Engineers of AIME, SPE 5544, 11 p. Gardner, G.H.F., Gardner, L.W. and Gregory, A.R Formation velocity and density: the diagnostic for stratigraphic traps. Geophysics, 39, GPT/IHS Mid-Norway Pressure Study, Non- Proprietary study by GeoPressure Technology and I H S Energy. Halim, M, F Thermal regimes of Malaysian sedimentary basins Volume: 78:7; Conference: AAPG International Conference and Exhibition, Kuala Lumpur (Malaysia), Aug Hermanrud, C., Wensaas, L., Teige, G.M.G., Norgård Bolås, H.M., Hansen, S. & Vik, E Shale Porosities from Well Logs on Haltenbanken (Offshore Mid-Norway) Show No Infl uence of Overpressuring, In Law, B.E., Ulmishek, G.F. & Slavin, V.I. (eds.), Abnormal pressures in hydrocarbon environments: AAPG Memoir 70, pp , The American Association of Petroleum Geologists. Hoesni, M.J., Swarbrick, R.E, and Goulty, N.R The Origins of overpressure in the Malay Basin AAPG Barcelona Conference, Spain. Hoesni, M.J The origin of overpressure in the Malay Basin and its infl uence on petroleum systems. Unpublished PhD thesis, University of Durham. Hoesni, M.J., Swarbrick, R.E, and Goulty, N.R The Signifi cance of Chemical Compaction in Modeling the Overpressure in the Malay Basin AAPG Hedberg Conference, The Hague, The Netherlands. Hubbert, M. K., Ruby, W. W The role of pore pressure in mechanics of mechanics of overthrust faulting. Geol. Soc.Am. Bull. 70, Kathara, K., 2006, Overpressure and shale properties: Stress unloading or smectite-illite transformation?. 76th annual International Meeting, SEG, Expanded Abstracts, paper PPP 1.2, p Lahann, R., McCarty, D. & Hsieh, J Infl uence of Clay Diagenesis on Shale Velocities and Fluid Pressure. OTC 13046, Offshore Technology Conference. Lahann, R., Impact of smectite diagenesis on compaction profi les and compaction equilibrium. In AADE Industry Forum on Pressure regimes in sedimentary basins and their prediction. Del Lago Resort, Lake Conroe, Texas. 2-4 September Madon, M Overpressure development in rift basins: an example from the Malay Basin, offshore Peninsular Malaysia Petroleum Geoscience v. 13; no. 2; p

10 Mann, D.M., and MacKenzie, A.S., Prediction Of Pore Pressures In Sedimentary Basins Marine and Petroleum Geology, v. 7, no. 1, p Mazzoni, R, Wahdan, T, Bassem, A and Ward, D.C Real-Time pore and fracture pressure prediction with FEWD in the Nile Delta SPE/IADC Mohamad, H, Jaini, N and Tajuddin, M. R 2006 drilling of deep-seated reservoir in high pressure regime in the north of malay basin. Petroleum Geology Conference and Exhibition 2006, 27-28th November, Kuala Lumpur Convention Center, Kuala Lumpur, Malaysia Norgård Bolås, H.M., Hermanrud, C. & Teige, G.M.G., The infl uence of stress regimes on hydrocarbon leakage. In: Boult, P. & Kaldi, J. eds. Evaluating fault and cap rock seals, AAPG Hedberg Series, 2, American Association of Petroleum Geologists. O Connor, S.A and Swarbrick, R.E, Where has all the pressure gone? Evidence from pressure reversals and hydrodynamic fl ow. First Break, v. 26 September 2008 Singh, I and Ford, C.H The occurrence, causes and detection of abnormal pressure in the Malay Basin. Offshore South East Asia Show, 9-12 February 1982, Singapore Skar, T., Van Balen, R.T., Arnesen, L. & Cloetingh, S Origin of overpressures on the Halten Terrace, offshore mid-norway: the potential role of mechanical compaction, pressure transfer and stress, In: Aplin, A.C., Fleet A.J. & Macquaker, J.H.S (eds.) Muds and Mudstones: Physical and Fluid Flow Properties, Geological Society Special Publication no. 158, pp , Geological Society of London. ISBN Swarbrick, Richard E., Osborne, Mark J., and Gareth S. Yardley, Comparison of overpressure magnitude resulting from the main generating mechanisms. AAPG Memoir 76, A. R. Huffman and G. L. Bowers, eds. Yusoff, W.I. and Swarbrick, R.E Thermal and Pressure Histories of the Malay Basin, Offshore Malaysia. AAPG International Conference and Exhibition, Kuala Lumpur, Malaysia, August 21-24, 1994 Osborne, M. J. and Swarbrick, R. E., Mechanisms which generate overpressure in sedimentary basins: a reevaluation. AAPG Bulletin, v 81, p Shariff Bin Kader, M Abnormal Pressure Study in the Malay and Penyu Basins: A Regional Understanding. Bulletin of the Geological Society of Malaysia, 36, p

11 Figure 1 - Location map for Malay Basin adapted from Madon (2007). Locations of all wells mentioned in the text are displayed.

12 Figure 2 - Well LA-3, Malay Basin (Yusoff and Swarbrick (1994). Wireline Formation Test (WFT) data (blue ovals) refi nes an overburdenparallel shale pore pressure profi le, characteristic of overpressure generated by disequilibrium compaction (black line). Sharp pressure transition zone below 100oC suggests additional overpressure generated by secondary processes (based on geothermal gradient of 51.8oC/km, Halim (1994). See text for discussion.

13 Figure 3 - Typical Velocity vs. Density signatures and their associated, causal mechanisms of overpressure generation (from Hoesni, 2004).

14 Figure 4 - Velocity vs. density data plotted for Well A. Estimated borehole temperatures also plotted. Deviation is observed from the primary compaction curve of Gardner (red line) and Bowers (2001) (blue line) for shales at temperatures of 120oC, although more signifi cantly at 160oC. This signature of increasing velocity and density is identifi ed by Hoesni et al. (2007) as suggestive of chemical compaction/cementation effects.

15 Figure 5 - Velocity vs. density data plotted for Well B. Deviation is observed from the primary compaction curve of Gardner (red line) and Bowers (2001) (blue line) for shales at temperatures of 120oC. This signature of increasing velocity and density is identifi ed as suggestive of load transfer effects (refer to Figure 3).

16 Figure 6 - An example of velocity/density behavior that requires load transfer interpretation in Gulf of Mexico. Solid blue line represents the primary compaction curve. In this case, the load transfer (or unloading termed by the author) shift (orange squares) occurred within the smectite-illite reaction window (Lahann, 2002).

17 Figure 7 - Hydrostatic pressure (light blue), contribution to overpressure from recent rapid burial (light blue to dark blue) and contribution to overpressure from load transfer (dark blue to purple) for Lange Formation shales, Halten Terrace, Mid-Norway. Reservoir overpressures, as measured by WRT data in encased shales in the Lange Formation, are substantially higher than could have been created by burial-related processes alone. OPTIMISE SUCCESS THROUGH SCIENCE