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Integrated analysis of well logs and seismic data to estimate gas hydrate concentrations at Keathley Canyon, Gulf of Mexico
Marine and Petroleum Geology 25 (2008) 924–931
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Integrated analysis of well logs and seismic data to estimate gas hydrate concentrations at Keathley Canyon, Gulf of Mexico M.W. Lee, T.S. Collett U.S. Geological Survey, Box 25046, MS-939, Denver Federal Center, Denver, CO 80225, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 January 2007 Received in revised form 1 May 2007 Accepted 28 September 2007
Accurately detecting and quantifying gas hydrate or free gas in sediments from seismic data require downhole well-log data to calibrate the physical properties of the gas hydrate-/free gas-bearing sediments. As part of the Gulf of Mexico Joint Industry Program, a series of wells were either cored or drilled in the Gulf of Mexico to characterize the physical properties of gas hydrate-bearing sediments, to calibrate geophysical estimates, and to evaluate source and transport mechanisms for gas within the gas hydrates. Downhole acoustic logs were used sparingly in this study because of degraded log quality due to adverse wellbore conditions. However, reliable logging while drilling (LWD) electrical resistivity and porosity logs were obtained. To tie the well-log information to the available 3-D seismic data in this area, a velocity log was calculated from the available resistivity log at the Keathley Canyon 151-2 well, because the acoustic log or vertical seismic data acquired at the nearby Keathley Canyon 151-3 well were either of poor quality or had limited depth coverage. Based on the gas hydrate saturations estimated from the LWD resistivity log, the modified Biot–Gassmann theory was used to generate synthetic acoustic log and a synthetic seismogram was generated with a fairly good agreement with a seismic profile crossing the well site. Based on the well-log information, a faintly defined bottom-simulating reflection (BSR) in this area is interpreted as a reflection representing gas hydrate-bearing sediments with about 15% saturation overlying partially gas-saturated sediments with 3% saturation. Gas hydrate saturations over 30–40% are estimated from the resistivity log in two distinct intervals at 220–230 and 264–300 m below the sea floor, but gas hydrate was not physically recovered in cores. It is speculated that the poor recovery of cores and gas hydrate morphology are responsible for the lack of physical gas hydrate recovery. Published by Elsevier Ltd.
Keywords: Gas hydrate Elastic velocities Synthetic acoustic log Saturation VSP
1. Introduction Gas hydrate, an ice-like compound of natural gas and water, is widespread in the Gulf of Mexico (GOM) (Milkov and Sassen, 2001), particularly on the continental slope with water depths in the range of 540–2000 m. The geology of northern GOM is complicated due to Mesozoic salt tectonics and numerous faults and fracture zones provide migration pathways for gas, which becomes gas hydrate under suitable conditions of pressure and temperature (Cooper and Hart, 2003). One objective of the DOE–Chevron Joint Industry program (JIP) for Methane Hydrates is the development of seismic analysis methods pertinent to gas hydrate-bearing sediments (GHBS). During the 2005 JIP field expedition, drilling, coring and logging were conducted to test, validate and adjust various models developed during earlier phases of the program. This paper focuses on analysis of well logs from two holes (KC 151-2 and KC 151-3) and
E-mail address: [email protected] (M.W. Lee). 0264-8172/$ – see front matter Published by Elsevier Ltd. doi:10.1016/j.marpetgeo.2007.09.002
coincident seismic data collected at Keathley Canyon (KC) lease block 151 site, one of two sites selected for drilling. One objective at KC 151 was to analyze possible gas hydrate presence associated with a BSR at a location lacking focused flux features. Gas hydrate formation changes the physical properties of sediments, leading to increased seismic velocities and electrical resistivity for example. Acoustic logs provide critical information to link the well information and the physical properties of sediments inferred from the recorded logs to the regional seismic data. Due to the adversely enlarged borehole conditions, only degraded acoustic logs were acquired in the KC 151-3 well. To link the well information to the 3-D seismic data in this area, a synthetic acoustic log was calculated from the resistivity log using the modified Biot– Gassmann theory (Lee, 2002), hereafter referred to as the BGTL, with the gas hydrate saturations estimated from the electrical resistivity log. Generating acoustic velocities from resistivities is not a new concept. Faust (1951) developed an empirical formula to relate resistivity to velocity using Archie’s relationship, which is thought to be applicable in permeable formations. There are many other
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empirical formulas, including Rudman et al. (1975), Brito Dos Santos et al. (1988), and Worthington (1991). All these empirical methods are applicable to relate velocity and resistivity for watersaturated sediments. Lee (1999) developed a method of relating resistivity to velocity of gas hydrate-bearing sediment by explicitly inserting the amount of gas hydrate independently estimated from the resistivity log into the acoustic equation and demonstrated that this method performs better than empirical relations. Lee (2005) applied this method to generate velocity logs for the permafrost in the North Slope of Alaska by estimating ice saturation from the resistivity log into the BGTL. This paper shows how to calculate an acoustic velocity log at the KC 151-2 well using the method described in Lee (2005). It also presents estimates of gas hydrate saturations at the KC 151-2 well, identifies key seismic horizons in the seismic section, and finally describes the nature of the BSR at the KC 151-2 well site.
2. Log analysis 2.1. Descriptions of logs The primary goal of this study is to predict P-wave velocity from the resistivity log at the KC 151-2 well to generate a synthetic seismogram, which can then be used to tie well information to 3-D seismic data. The logs used in this study are natural gamma, bulk density, electrical resistivity, and the acoustic logs collected by the Dipole Shear Sonic Imager (DSI) tool. Sediment porosities were derived from the bulk density logs with a grain density of 2718 kg m3 and water density of 1050 kg m3 assuming a two-component system (water and grains). The densities adopted here differ by less than a few percent from those used by Cook et al. (2008); Dugan (2008), and Dai et al. (2008) and should not affect the degree to which the results and interpretations of various studies can be compared. The grain density adopted here is the average gain density measured from recovered cores (Winters et al., 2008). The corederived porosities agree well with bulk density log porosities, but neutron log porosities are generally higher than the core-derived porosities. Bulk density log porosities shallower than 120 m below sea floor (mbsf) are degraded due to an enlarged borehole conditions. Therefore, porosities for depths less than 117 mbsf were replaced by calculated porosities, which were estimated from the resistivity log assuming the sediment pores are saturated with water. The porosity correction due to the presence of gas hydrate is accomplished simultaneously with the estimation of gas hydrate. Shale volumes were calculated from the gamma log by using the formula appropriate for Tertiary clastics (Western Atlas International Inc., 1995). DSI logs were acquired at well KC 151-3, close to KC 151-2, and the P-wave acoustic log from KC 151-3 provides some useful information. However, the quality of the S-wave log is not good, and there appears to be many cycle skips in the S-wave velocities. Fig. 1 shows the measured P-wave and S-wave velocities between 200 and 240 mbsf at KC 151-3. The shear-wave velocities are never less than about 280 m s1 regardless of porosity or P-wave velocity, suggesting an inaccurate threshold for S-wave arrival time. In many places in the log, an increase of P-wave velocity accompanies a decrease in S-wave velocity, implying cycle skips in the S-wave log. Also the P-wave interval velocities from the DSI log are much smaller than those derived from the vertical seismic profile data at the same well, shown also in Fig. 1. Because of a poor quality of DSI logs at KC 151-3 well, the primary use of the DSI log at the KC 151-3 well was to derive the BGTL parameter l, which is mostly dependent on the degree of sediment consolidation (Lee, 2002).
Fig. 1. Dipole Shear Sonic Imager (DSI) velocity logs for intervals between 200 and 240 mbsf at the KC 151-3 well. P-wave interval velocities derived from the vertical seismic profile data at the same well are also shown.
2.2. Baseline parameters and estimation of gas hydrate saturations Gas hydrate saturations from the resistivity log were estimated using the Archie equation (Archie, 1942). To estimate gas hydrate saturation, resistivities of water-saturated sediments are calculated using variable Ra, which is Ra ¼ aRw, where a is the Archie parameter and Rw is the resistivity of connate water. The heavy red line in Fig. 2A, which is averaged over 15 m, is the calculated resistivity of the water-saturated sediments (Ro), which is given by the following equation:
Ro ¼
aRw
fm
¼
Ra
f
1:6
¼
0:4 0:17d=400
f1:6
(1)
where d is the subbottom depth in meters. According to Eq. (1), the apparent resistivity decreases linearly with depth. It is assumed that Ro calculated with Eq. (1) serves as an accurate baseline for the resistivity data at the KC 151-2 well. In estimating gas hydrate saturations, resistivities higher than the Ro baseline resistivities are assumed to be due to gas hydrate. The gas hydrate saturations estimated from the resistivity using Archie’s equation with m ¼ 1.6 and Ra as given by Eq. (1) are shown in Fig. 3. Although, nuclear magnetic resonance (NMR) porosities were measured at the KC 151-2 well, these porosities were not used in estimating gas hydrate saturations in this study because the formation is dominantly silty clays. In the presence of clay bound water with fast relaxation times, the apparent NMR porosities yield overestimates of gas hydrate saturations (Collett et al., 2003). When the pore space is filled with free gas instead of gas hydrate, the saturations calculated from the resistivity assuming gas hydrate pore fill are slightly different from that calculated assuming free gas owing to the density difference between gas and gas hydrate. Lee (2005) derived an approximation formula for
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A
B
Fig. 2. A) Measured resistivity with depth at the Keathley Canyon 151-2 well is shown as a thin line. The thick line is the baseline resistivity calculated using m ¼ 1.6 with Ra ¼ 0:4 0:17d=400 Um. The inset shows the detailed resistivity near the presumed BGHS or the location of bottom-simulating reflector (BSR). (B) Calculated resistivity of connate water (Rw) at Keathley Canyon 151-3 well using the Arps (1953) formula with measured temperature and salinity (dots) and estimated Rw at the Keathley Canyon 151-2 well from Ra as given in Eq. (1) with m ¼ 1.6 and a ¼ 1.6. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
estimating free-gas saturations from the saturations estimated by assuming gas hydrate pore fill.
2.3. Predicting P-wave velocity from resistivity and porosity logs When P-wave velocity measurements are lacking, P-wave velocities are often predicted from resistivity logs by using a leastsquares method (Rudman et al., 1975). However, as shown in Lee (1999, 2005), predicting velocities by explicitly inserting gas
hydrate saturations estimated from the resistivity into the acoustic equation is a more accurate method. To predict velocities from the resistivity data in this study, the BGTL parameter l must be known. This parameter can be estimated if porosity and velocity of water-saturated sediments (or baseline velocity) are known. Because there was no porosity log at the KC 151-3 well, porosities derived from a least-squares fit to the density porosity measured at the KC 151-2 well were used to estimate the BGTL parameter l ¼ 1.5. A brief description of the BGTL is given in Appendix A. The method of predicting velocities from the resistivity is: 1. Estimate gas hydrate saturations from the resistivity and porosity logs with m ¼ 1.6 in the Archie equation and apparent resistivity described previously (Eq. 1). 2. Using the gas hydrate saturations, porosity, and clay volume content, calculate P-wave and S-wave velocities from the BGTL (Lee, 2002) with l ¼ 1.5 for sediments above the base of gas hydrate stability (BGHS) 3. To compute velocities for sediments below the BGHS, use the Biot–Gassmann theory (BGT) with parameters calculated from the BGTL with free-gas saturations as shown in Lee (2004).
Fig. 3. Gas hydrate saturation (smoothed over 21 points or 3 m) estimated from the resistivity log using m ¼ 1.6 with Ra ¼ 0:4 0:17d=400 Um.
Fig. 4A shows P-wave and S-wave velocities predicted from resistivity and density logs with gas hydrate saturations estimated from the resistivity log (shown in Fig. 3) using the BGTL under the assumption that there is no free gas. If it is assumed that the location of the GHPB (or BSR) is at 392 mbsf, the free-gas saturations below the BSR are slightly less than those shown in Fig. 3 and the predicted P-wave velocity assuming free gas below 392 mbsf is shown in Fig. 4B. The P-wave velocities for partially gas-saturated sediments (PGSS) are calculated at logging frequency of w10 kHz using the White (1975) theory. Because of the velocity dispersion for the PGSS, the P-wave velocities at seismic frequencies below 100 Hz are much smaller than those shown in Fig. 4B.
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A
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B
Fig. 4. Resistivity-derived velocities at the Keathley Canyon 151-2 well by explicitly inserting the gas hydrate saturation into the modified Biot–Gassmann theory (BGTL) with l ¼ 1.5, volume clay content calculated from the gamma log, and differential pressure with depth under the normal pressure regime. (A) P-wave and S-wave velocities assuming all saturants are gas hydrate. (B) P-wave velocities assuming that free gas is present below 392 mbsf.
3. Synthetic seismogram In order to relate the resistivity-derived velocity log (Fig. 4) to seismic profiles, a synthetic vertical seismic profile (VSP) for the KC 151-2 well is generated and shown in Fig. 5. In generating the synthetic VSP, it is assumed that there is no BGHS at the depth 392 mbsf. Because the resistivity responses from free gas and gas hydrate are the same, it is not possible to infer free gas below the BGHS solely on the basis of the resistivity log. If it is assumed that there is a BSR at depth 392 mbsf, the synthetic VSP accounting for the free gas below BSR is almost identical to the one shown in Fig. 5 except for a larger amplitude at the BSR level. This is because the P-wave velocity from the PGSS at seismic frequencies is much smaller than that for GHBS at the logging frequency. A regional unconformity at w100 mbsf is represented by a strong peak in the synthetic. Two large peak–trough combinations clearly indicate the zones of high gas hydrate saturated intervals, and the assumed BSR is indicated as a reverse-polarity reflection.
1680 m s1, whereas the average velocity at the KC 151-2 well is 1790 m s1. The average velocity of the VSP data in this interval is 1740 m s1. Therefore, the average velocity of the measured velocity at the KC 151-3 well is about 4% smaller than the average velocity of
4. Discussion 4.1. Predicted and measured velocities The P-wave velocities measured at the KC 151-3 well using the DSI tool are shown in Fig. 6 along with the interval velocities derived from the zero-offset VSP data. The well log P-wave velocities at the KC 151-3 well are w4% less than those from the VSP. The lower part of Fig. 6 shows the calculated velocities from the resistivity log at the KC 151-2 well. Distinct velocity markers observed at both wells are correlated with dotted lines. Note that the high velocity measured near 220 mbsf at the KC 151-3 well agrees well with the predicted velocity at the KC 151-2 well. However, high velocities near 290 mbsf at the KC 151-2 well are missing at the KC 151-3 well. Overall, the measured velocities at the KC 151-3 well are less than the predicted velocities at the KC 151-2 well. The average velocity at the KC 151-3 well (depth range from 123 to 331 mbsf) is
Fig. 5. Synthetic vertical seismic profile generated at the Keathley Canyon 151-2 well using the resistivity-derived P-wave velocities (Fig. 4) and measured density.
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Fig. 7. Multichannel seismic line KC 1 (Hutchinson et al., in press) crossing the Keathley Canyon 151-2 well, with a synthetic seismogram inserted.
Fig. 6. Comparison of P-wave velocities measured at the Keathley Canyon 151-3 well and derived from the resistivity logs at the Keathley Canyon 151-2 well.
VSP data, whereas the average velocity predicted at the KC 151-2 well is about 3% higher. The difference between measured and predicted velocities is significant considering that the distance between the two wells is small (w10 m) and that the geology appears to be uniform on the basis of seismic data. The difference may be caused by intra-site variability, quality of well logs, or the presence of free gas. The gas hydrate occurrence appears to be controlled by fractures (possibly through-going faults) as seen on the resistivity-at-bit images from LWD data in KC 151-2 well. It is not uncommon to have correlation problems between wells. The resistivity log was acquired while drilling (LWD log), and the depth penetration is deeper than velocity or density logs. The adverse effects such as borehole washouts could have been minimal for the LWD logs. Therefore, the predicted velocities could be more accurate than the measured velocities. Also measured P-wave log velocities may be affected by free gas associated with the dissociation of gas hydrate. Because of the inaccurate S-wave velocities, it is not possible to evaluate the presence of free gas with the wireline velocity logs. However, it is speculated that the average P-wave velocity being lower than the VSP-derived velocity could reflect the impact of a small amount of free gas on velocity, as demonstrated for the velocity logs at the Hydrate Ridge, offshore Oregon (Lee and Collett, 2005). 4.2. Synthetic seismogram and 3-D seismic A seismic line crossing the KC 153-2 well site is shown in Fig. 7 with a synthetic seismogram generated from the resistivity-derived velocity log (the top part of synthetic VSP shown in Fig. 5). The
match between the synthetic seismogram and seismic data is fairly good. Note that the interpreted BSR on the seismic section matches the assumed BSR reflection shown in the synthetic. A good match of BSR reflections between seismic and synthetic seismogram provides confidence in the predicted P-wave velocity and the gas hydrate saturations estimated from the resistivity. The seismic response of the zone with high gas hydrate saturations near the depth 225 m (depth between 220 and 230 mbsf and designated the ‘‘upper zone’’) is indicated as strong peaks both in the synthetic and 3-D seismic data. The average gas hydrate saturation in this interval is w25%. This reflection amplitude decreases as the reflection event moves in the down-dip direction. However, the synthetic response for the zone with high gas hydrate saturations near the depth 275 m (between 264 and 298 mbsf, with four highly saturated intervals and designated the ‘‘lower zone’’) is much stronger than that in the seismic data. The average gas hydrate saturation for the lower zone is w24%. The measured velocity at the KC 151-3 well for the lower zone is much lower than the predicted velocity (Fig. 6) and predicts much lower amplitudes similar to those shown in the 3-D seismic data. The VSP data acquired at the KC 151-3 well also indicate weak reflections for this interval. The discrepancy between the two wells and between synthetic and surface seismic data is likely due to a localized small anomalous zone at the KC 151-2 well, an anomaly too small to be detectable in the surface seismic or VSP data.
4.3. Nature of the BSR at Keathley Canyon 151-2 Gas hydrate and free gas are both electrical insulators, so there is no difference in the response to the formation resistivity. Therefore, locating the BSR based only on the resistivity without knowing P-wave velocity entails uncertainty. The insert in Fig. 2A shows detailed resistivities near the assumed BSR. The shaded region is a high resistivity zone compared to the adjacent depth intervals, and the BGHS can be anywhere inside the shaded area at the KC 151-2 well site.
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The resistivity logs acquired at the Hole 994D and 995B during Ocean Drilling Program (ODP) Leg 164 at the Blake Outer Ridge, southeastern U.S., indicate sharp decreases of resistivities at the BGHS (Fig. 5 of Collett and Ladd (2000)). At these sites, the resistivities at and below BGHS follow the baseline resistivities fairly well. If the resistivity characteristic of BGHS at the KC 153-2 is similar to that at Blake Outer Ridge, the BGHS is possibly near 392 mbsf. The average free-gas saturation between 392 and 400 mbsf is w3%. The average gas hydrate saturation of high resistivity zone between 386 and 392 mbsf is w15% with an average velocity of 1940 m s1. Therefore, the BSR at the KC 152-3 well site appears to be caused by the BGHS with about 15% saturation overlying partially gas-saturated sediments with about 3% saturation.
4.4. Gas hydrate saturations and gas hydrate evidence Although the gas hydrate saturations estimated from the resistivity log indicate that saturations approach as much as 40% in several thin intervals (Fig. 3), Claypool (2006) indicated that the presence of gas hydrate was not fully verified in the KC 151-2 well except for indirect evidence of the gas hydrate such as cold spots, moussey sediment texture, and anomalously low pore water salinity (Kastner et al., 2008). Also, several recovered pressure cores indicated gas concentrations exceeding normal solubility, although no gas hydrate was visually identified (Claypool, 2006). During ODP Leg 164, gas hydrates were recovered as nodules and veins at the Blake Ridge. The well logs at the Blake Ridge indicate that the maximum gas hydrate saturation is w20% of the pore volume (Collett and Ladd, 2000), which is equivalent to w10% bulk volume of the sediment. An average porosity in the upper zone at the KC 151-2 well is w40%, so w16% of bulk volume is gas hydrate. Although the bulk volume of gas hydrate at the KC 151-2 is greater than that at the Blake Ridge (but water depth and BGHS are much deeper at the Blake Ridge than at the KC 151-2 area), gas hydrate was not recovered at the KC 151-2 well site. One interpretation is that gas hydrate saturation is overestimated at the KC 151-2 well and that there is not much gas hydrate. This means that the parameters for the Archie equation are incorrect; either the apparent resistivity is too low or the Archie cementation parameter is too small. However, gas hydrate saturations estimated from the resistivity log using the Humble equation (Winsauer et al., 1952) are w30% for high gas hydrate saturated intervals. This still yields an estimate of w12% bulk volume of the sediment being occupied by gas hydrate at KC 151-2. The resistivity of connate water can be calculated using the Arps formula (Arps, 1953) with salinity and temperature. The Arps formula is Rw2 ¼ Rw1 ðT1 þ 7Þ=ðT2 þ 7Þ, where Rw1 and Rw2 are water resistivities at Fahrenheit temperatures at T1 and T2, matched by laboratory measurement and subsurface formation conditions, respectively. The calculated Rw (dots in Fig. 2b) is w0.275 Um near the seabed and is w0.15 Um at the subbottom depth of 400 m when using the seabed temperature of 4.79 C, a thermal gradient of 30 mK m1 (Hutchinson et al., in press), and pore water salinity measured from the core (Kastner et al., 2008). The estimated Rw (solid line in Fig. 2b) from the Ra in Eq. (1), with a ¼ 1.6, is Rw ¼ 0.25 Um at the seabed and Rw ¼ 0.14 Um at d ¼ 400 m. Except at shallow depths (less than 50 mbsf), estimated resistivities of connate water from the Archie analysis agree well with the Rw calculated using the Arps formula. Therefore, unless there are significant errors in the measurements, the gas hydrate saturations estimated from the resistivity log seem to be reasonable. In other words, the high gas hydrate saturations are not likely due to the error in the data analysis.
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What are other possible causes of not physically observing the gas hydrate in cores at intervals with high resistivities, particularly at the upper zone (220–230 mbsf)? The reasons include: (1) High resistivity caused by low porosity. Archie’s equation indicates that the formation resistivity of water-saturated sediment is greater than 3 Um if porosity is less than about 10% when using m ¼ 1 with Ra ¼ 0.45 Um or less than 20% when using m ¼ 1 and Ra ¼ 0.55 Um. However, the average porosity for this interval is w41% and not much different from that of adjacent intervals with no or only small degrees of gas hydrate saturation. Therefore, the argument that low porosity may be responsible for the log measured high resistivities is ruled out. (2) High resistivity is caused by low salinity. Salinity of connate water significantly affects the measured formation resistivity. If the pore water is fresh in this interval, the resistivity of the formation would be high without any gas or gas hydrate in the pore. The pore water analysis of cores in this interval indicates elevated pore water salinities exceeding 50 ppt. Using the Arps formula with an in situ temperature of 10 C and salinity of 50 ppt, the resistivity of pore water is w0.2 Um. The calculated formation resistivity of this interval assuming water saturation is w0.85 Um when using the Humble equation with calculated Rw (or 0.3 Um if we use m ¼ 1), and this value is much less than the observed anomalous resistivity, which is greater than 2–3 Um. To have greater than 2 Um resistivity, the salinity of pore water should be much less than the measured salinity. Therefore, variations in pore water salinity cannot explain the high resistivities in this interval. (3) High resistivity is caused by free gas instead of gas hydrate. Even inside the gas hydrate stability zone, free gas can co-exist with gas hydrate (Guerin et al., 1999; Lee and Collett, 2005). So it can be assumed that this interval contains free gas. If this is the case, the P-wave velocities should be much less than those shown in Fig. 4, and the BSR reflection from the synthetic would not agree with the BSR interpreted from the 3-D seismic data. Also, the measured velocity at the nearby KC 151-3 well indicates high velocities in this interval. Therefore, free gas may not be a possible primary cause of high resistivity of this interval. (4) The morphology of gas hydrate in the pore space and extended time of core recovery may be responsible. If the gas hydrate is disseminated in the pore space without forming large nodules or veins, the gas hydrate in the core may be easily dissociated during the long trip to the surface. Consequently, only indirect evidence for the presence of gas hydrate (e.g., cold spots or moussey sediment texture) would remain in the core. From an analysis of cores at the Hydrate Ridge offshore Oregon, Milkov et al. (2004) noted that the detection of finely disseminated gas hydrate in deep marine sediments depends mainly on indirect evidence because of rapid decomposition of the gas hydrate upon core recovery, and this observation supports our speculation. However, in order to substantiate this speculation, numerical modeling of dissociation of gas hydrate due to changing temperature and pressure of core during the recovery should be performed. 5. Conclusions The LWD resistivity log at the KC 151-2 well enabled us to predict accurate P-wave velocities and to generate a synthetic seismogram which agrees well with a 3-D seismic line. A faint BSR in this area is possibly caused by gas hydrate-bearing sediments with w15% saturation overlying partially gas-saturated sediments with very low saturations of w3%. The analysis of resistivity log
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indicates that the gas hydrate saturations reach w30–40% of the pore space within two zones: an upper zone (220–230 mbsf) and lower zone (264–298 mbsf). The amplitude of the synthetic seismogram for the upper zone agrees well with the amplitude in the 3-D seismic data. But the amplitude of lower zone does not agree with 3-D seismic data, implying that the lower interval may be a small localized anomalous zone and not continuous across the site. The absence of physically observed gas hydrate in cores with high gas hydrate saturations estimated from the resistivity log bring us to an enigma in interpreting both cores and logs. It is speculated that the gas hydrate morphology, which is finely disseminated in the pore space or in the fractures without large nodules or veins, in combination with poor core recovery, is responsible for the absence of observed gas hydrate in the cores.
Appendix A
G ¼ 0:9552 þ 0:0448 eCv =0:06714 0:18S1=2
(A6)
where p is differential pressure in MPa, l is a constant related to the degree of consolidation, Cv is the decimal clay volume content of the grains, and S is the decimal gas hydrate saturation in the pore space. For a detailed discussion of parameters n and G, consult Lee (2002). The parameter l is given by the following form when the rate of porosity change with respect to differential pressure is known (Lee, 2002):
l ¼ 1:0 þ 4:953 e5212vf=vp
(A7)
where p is in MPa. For unconsolidated sediments, the rate of porosity change with respect to differential is large (large negative number) and l becomes small. Generally l for the unconsolidated sediments varies between 1 and 3 and is treated as a free parameter to fit the measurements.
Modified Biot–Gassmann theory (BGTL) References The essence of the BGTL is to derive the bulk modulus of the sediments from the Biot–Gassmann theory (BGT) and shear modulus of the sediment from the velocity ratio in the form of Vs ¼ Vp Gað1 fÞn , where Vp is the P-wave velocity, Vs is the S-wave velocity, ais the Vs/Vp of the grains, and fis the porosity. G and n are parameters depending on differential pressure, consolidation, and clay content. The bulk modulus of sediments (k) is given by the following equation (the Biot–Gassmann theory): 2
k ¼ kma ð1 bÞ þ b M
(A1)
where
1 ðb fÞ f þ ¼ kfl M kma and the shear modulus is given by the following equation (BGTL formulation):
mma G2 ð1 fÞ2n k h i. kma þ 4mma 1 G2 ð1 fÞ2n 3
m ¼
(A2)
In Eqs. (A1) and (A2), kma,mma ,kfl ,f, and b are the bulk modulus of grains, the shear modulus of the grains, the bulk modulus of the fluid, porosity, and the Biot coefficient, respectively. Elastic velocities of water-saturated sediments can be computed from the elastic moduli by the following formulas:
Vp ¼
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi k þ 4m=3
r
and
Vs ¼
rffiffiffi
m r
(A3)
where r is density of the formation given by r ¼ ð1 fÞrma þ fr,and rma and rfl are the grain density and pore fluid density, respectively. For soft rocks or unconsolidated sediments, the following Biot coefficient is used (Lee, 2002):
b¼
184:05 þ 0:99494 1 þ expfðf þ 0:56468Þ=0:10817g
(A4)
The exponent n is given by
h n ¼ 10ð0:4260:235 log10 and G is given by
pÞ
i. l
(A5)
Archie, G.E., 1942. The electrical resistivity log as an aid in determining some reservoir characteristics. Petr. Trans. AIME. 146, 54–62. Arps, J.J., 1953. The effect of temperature on the density and electrical resistivity of sodium chloride solutions. Petr. Trans. AIME 198, 327–330. Brito Dos Santos, W.L., Ulrych, T.J., De Lima, O.A.L., 1988. A new approach for deriving pseudovelocity logs from resistivity logs. Geophys. Prospect. 36, 83–91. Claypool, G.E. (Ed.), 2006. Gulf of Mexico Gas Hydrate Joint Industry Project (GOMJIP), 2005. The Cruise of the Drilling Vessel Uncle John, Mobile, Alabama to Galveston, Texas, Atwater Valley Blocks 13/14 and Keathley Canyon Block 151, 17 April to 22 May 2005.. Collett, T.S., Goldberg, D.S., Janik, A., Guerin, G., 2003. Downhole log assessment of Hydrate Ridge. EOS, Trans. AGU 84 (46) OS51C-0876. Collett, T.S., Ladd, J., 2000. Detection of gas hydrate with downhole logs and assessment of gas hydrate concentrations (saturations) and gas volumes on the Blake Ridge with electrical resistivity log data. In: Paull, C.K., Matsumoto, R., Wallace, P.J., Dillon, W.P. (Eds.), Proc. ODP, Sci. Res., 164. TAMU, College Station, TX, pp. 179–191. Cook, A.E., Goldberg, D., Kleinberg, R.L., 2008. Fracture-controlled gas hydrate systems in the northern Gulf of Mexico. Mar. Petr. Geol. 25, 932–941. Cooper, A.K., Hart, P.E., 2003. High-resolution seismic-reflection investigation of the northern Gulf of Mexico gas-hydrate-stability zone. Mar. Petr. Geol. 19, 1275–1293. Dai, J., Banik, N., Gillespie, D., Dutta, N., 2008. Exploration for gas hydrates in the deepwater northern Gulf of Mexico: part II. Model validation by drilling. Mar. Petr. Geol. 25, 830–844. Dugan, B., 2008. Fluid flow in the Keathley Canyon 151 mini-basin, northern Gulf of Mexico. Mar. Petr. Geol., 25, 906–918. Faust, L.Y., 1951. Seismic velocity as function of depth and geologic time. Geophysics 16, 192–206. Guerin, G., Goldberg, D., Meltser, A., 1999. Characterization of in situ elastic properties of gas hydrate-bearing sediments on the Blake Ridge. J. Geophys. Res. 104, 17,781–17,795. Hutchinson, D.R., Hart, P.E., Ruppel, C.D., Snyder, F. and Dugan, B. Seismic and thermal characterization of a Bottom Simulating Reflection in the northern Gulf of Mexico. In: Collett, T.S., Johnson, A., Knapp, C., Boswell, R. (Eds.), Natural Gas Hydrates: Energy Resources, Potential and Associated Geologic Hazards. AAPG Memoir 89 (DVD-ROM), in press. Kastner, M., Claypool, G., Robertson, G., 2008. Geochemical constraints on the origin of the pore fluids and gas hydrate distribution at Atwater Valley and Keathley Canyon, Northern Gulf of Mexico. Mar. Petr. Geol. 25, 860–872. Lee, M.W., 1999. Methods of generating synthetic acoustic logs from resistivity logs for gas-hydrate-bearing sediments. U.S. Geol. Surv. Bull. 2170. http://geology.cr. usgs.gov/pub/bulletins/b2170/ 11pp. Lee, M.W., 2002. Modified Biot–Gassmann theory for calculating elastic velocities for unconsolidated sand consolidated sediments. Mar. Geophys. Res. 23, 403–412. Lee, M.W., 2004. Elastic velocities of partially gas-saturated unconsolidated sediments. Mar. Petr. Geol. 21, 641–650. Lee, M.W., 2005. Well log analysis to assist the interpretation of 3-D seismic data at Milne Point, North Slope of Alaska. U.S. Geol. Surv. Sci. Inv. Rept. 2005–5048. http://pubs.usgs.gov/sir/2005/5048 18pp. Lee, M.W., Collett, T.S., 2005. Gas hydrate and free gas saturations estimated form velocity logs on Hydrate Ridge, offshore Oregon, U.S.A.. Available from: In: Trehu, A.M., Bohrmann, G., Torres, M.E., Colwell, F.S. (Eds.), Proc. Ocean Drill. Prog. Sci. Res., 204 http://www-odp.tamu.edu/publications/204_SR/103/103. PDF. Milkov, A.V., Claypool, G.E., Lee, J.-Y., Torres, M.E., Borowski, W.S., Tomaru, H., Sassen, R., Long, P.E., ODP Leg 204 Scientific Party, 2004. Subsurface gases
M.W. Lee, T.S. Collett / Marine and Petroleum Geology 25 (2008) 924–931 indicate gas hydrate occurrence in marine sediments at southern Hydrate Ridge offshore Oregon. Org. Geochem. 35, 1067–1080. Milkov, A.V., Sassen, R., 2001. Two-dimensional modeling of gas hydrate decomposition in the northwestern Gulf of Mexico: significance to global change assessment. Global Planet. Change 36, 31–46. Rudman, A.J., Whaley, J.F., Blakely, R.F., Biggs, M.E., 1975. Transform of resistivity to pseudovelocity logs. AAPG Bull. 59, 1151–1165. Western Atlas International Inc., 1995. Introduction to Wireline Log Analysis. Western Atlas International Inc. (Chapter 4)312 pp
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White, J.E., 1975. Computed seismic speeds and attenuation in rocks with partial gas saturation. Geophysics 40, 224–232. Winters, W.J., Dugan, B., Collett, T.S., 2008. Physical properties of sediments from Keathley Canyon and Atwater Valley, JIP Gulf of Mexico gas hydrate drilling program. Mar. Petr. Geol. 25, 896–905. Winsauer, W.O., Shearin Jr., H.M., Masson, P.H., Williams, M., 1952. Resistivity of brine-saturated sands in relation to pore geometry. AAPG Bull. 36, 253–277. Worthington, P.F., 1991. The integrity of pseudo-acoustic transforms. Sci. Drill. 2, 279–286.
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Marine and Petroleum Geology journal homepage: www.elsevier.com/locate/marpetgeo
Integrated analysis of well logs and seismic data to estimate gas hydrate concentrations at Keathley Canyon, Gulf of Mexico M.W. Lee, T.S. Collett U.S. Geological Survey, Box 25046, MS-939, Denver Federal Center, Denver, CO 80225, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 January 2007 Received in revised form 1 May 2007 Accepted 28 September 2007
Accurately detecting and quantifying gas hydrate or free gas in sediments from seismic data require downhole well-log data to calibrate the physical properties of the gas hydrate-/free gas-bearing sediments. As part of the Gulf of Mexico Joint Industry Program, a series of wells were either cored or drilled in the Gulf of Mexico to characterize the physical properties of gas hydrate-bearing sediments, to calibrate geophysical estimates, and to evaluate source and transport mechanisms for gas within the gas hydrates. Downhole acoustic logs were used sparingly in this study because of degraded log quality due to adverse wellbore conditions. However, reliable logging while drilling (LWD) electrical resistivity and porosity logs were obtained. To tie the well-log information to the available 3-D seismic data in this area, a velocity log was calculated from the available resistivity log at the Keathley Canyon 151-2 well, because the acoustic log or vertical seismic data acquired at the nearby Keathley Canyon 151-3 well were either of poor quality or had limited depth coverage. Based on the gas hydrate saturations estimated from the LWD resistivity log, the modified Biot–Gassmann theory was used to generate synthetic acoustic log and a synthetic seismogram was generated with a fairly good agreement with a seismic profile crossing the well site. Based on the well-log information, a faintly defined bottom-simulating reflection (BSR) in this area is interpreted as a reflection representing gas hydrate-bearing sediments with about 15% saturation overlying partially gas-saturated sediments with 3% saturation. Gas hydrate saturations over 30–40% are estimated from the resistivity log in two distinct intervals at 220–230 and 264–300 m below the sea floor, but gas hydrate was not physically recovered in cores. It is speculated that the poor recovery of cores and gas hydrate morphology are responsible for the lack of physical gas hydrate recovery. Published by Elsevier Ltd.
Keywords: Gas hydrate Elastic velocities Synthetic acoustic log Saturation VSP
1. Introduction Gas hydrate, an ice-like compound of natural gas and water, is widespread in the Gulf of Mexico (GOM) (Milkov and Sassen, 2001), particularly on the continental slope with water depths in the range of 540–2000 m. The geology of northern GOM is complicated due to Mesozoic salt tectonics and numerous faults and fracture zones provide migration pathways for gas, which becomes gas hydrate under suitable conditions of pressure and temperature (Cooper and Hart, 2003). One objective of the DOE–Chevron Joint Industry program (JIP) for Methane Hydrates is the development of seismic analysis methods pertinent to gas hydrate-bearing sediments (GHBS). During the 2005 JIP field expedition, drilling, coring and logging were conducted to test, validate and adjust various models developed during earlier phases of the program. This paper focuses on analysis of well logs from two holes (KC 151-2 and KC 151-3) and
E-mail address: [email protected] (M.W. Lee). 0264-8172/$ – see front matter Published by Elsevier Ltd. doi:10.1016/j.marpetgeo.2007.09.002
coincident seismic data collected at Keathley Canyon (KC) lease block 151 site, one of two sites selected for drilling. One objective at KC 151 was to analyze possible gas hydrate presence associated with a BSR at a location lacking focused flux features. Gas hydrate formation changes the physical properties of sediments, leading to increased seismic velocities and electrical resistivity for example. Acoustic logs provide critical information to link the well information and the physical properties of sediments inferred from the recorded logs to the regional seismic data. Due to the adversely enlarged borehole conditions, only degraded acoustic logs were acquired in the KC 151-3 well. To link the well information to the 3-D seismic data in this area, a synthetic acoustic log was calculated from the resistivity log using the modified Biot– Gassmann theory (Lee, 2002), hereafter referred to as the BGTL, with the gas hydrate saturations estimated from the electrical resistivity log. Generating acoustic velocities from resistivities is not a new concept. Faust (1951) developed an empirical formula to relate resistivity to velocity using Archie’s relationship, which is thought to be applicable in permeable formations. There are many other
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empirical formulas, including Rudman et al. (1975), Brito Dos Santos et al. (1988), and Worthington (1991). All these empirical methods are applicable to relate velocity and resistivity for watersaturated sediments. Lee (1999) developed a method of relating resistivity to velocity of gas hydrate-bearing sediment by explicitly inserting the amount of gas hydrate independently estimated from the resistivity log into the acoustic equation and demonstrated that this method performs better than empirical relations. Lee (2005) applied this method to generate velocity logs for the permafrost in the North Slope of Alaska by estimating ice saturation from the resistivity log into the BGTL. This paper shows how to calculate an acoustic velocity log at the KC 151-2 well using the method described in Lee (2005). It also presents estimates of gas hydrate saturations at the KC 151-2 well, identifies key seismic horizons in the seismic section, and finally describes the nature of the BSR at the KC 151-2 well site.
2. Log analysis 2.1. Descriptions of logs The primary goal of this study is to predict P-wave velocity from the resistivity log at the KC 151-2 well to generate a synthetic seismogram, which can then be used to tie well information to 3-D seismic data. The logs used in this study are natural gamma, bulk density, electrical resistivity, and the acoustic logs collected by the Dipole Shear Sonic Imager (DSI) tool. Sediment porosities were derived from the bulk density logs with a grain density of 2718 kg m3 and water density of 1050 kg m3 assuming a two-component system (water and grains). The densities adopted here differ by less than a few percent from those used by Cook et al. (2008); Dugan (2008), and Dai et al. (2008) and should not affect the degree to which the results and interpretations of various studies can be compared. The grain density adopted here is the average gain density measured from recovered cores (Winters et al., 2008). The corederived porosities agree well with bulk density log porosities, but neutron log porosities are generally higher than the core-derived porosities. Bulk density log porosities shallower than 120 m below sea floor (mbsf) are degraded due to an enlarged borehole conditions. Therefore, porosities for depths less than 117 mbsf were replaced by calculated porosities, which were estimated from the resistivity log assuming the sediment pores are saturated with water. The porosity correction due to the presence of gas hydrate is accomplished simultaneously with the estimation of gas hydrate. Shale volumes were calculated from the gamma log by using the formula appropriate for Tertiary clastics (Western Atlas International Inc., 1995). DSI logs were acquired at well KC 151-3, close to KC 151-2, and the P-wave acoustic log from KC 151-3 provides some useful information. However, the quality of the S-wave log is not good, and there appears to be many cycle skips in the S-wave velocities. Fig. 1 shows the measured P-wave and S-wave velocities between 200 and 240 mbsf at KC 151-3. The shear-wave velocities are never less than about 280 m s1 regardless of porosity or P-wave velocity, suggesting an inaccurate threshold for S-wave arrival time. In many places in the log, an increase of P-wave velocity accompanies a decrease in S-wave velocity, implying cycle skips in the S-wave log. Also the P-wave interval velocities from the DSI log are much smaller than those derived from the vertical seismic profile data at the same well, shown also in Fig. 1. Because of a poor quality of DSI logs at KC 151-3 well, the primary use of the DSI log at the KC 151-3 well was to derive the BGTL parameter l, which is mostly dependent on the degree of sediment consolidation (Lee, 2002).
Fig. 1. Dipole Shear Sonic Imager (DSI) velocity logs for intervals between 200 and 240 mbsf at the KC 151-3 well. P-wave interval velocities derived from the vertical seismic profile data at the same well are also shown.
2.2. Baseline parameters and estimation of gas hydrate saturations Gas hydrate saturations from the resistivity log were estimated using the Archie equation (Archie, 1942). To estimate gas hydrate saturation, resistivities of water-saturated sediments are calculated using variable Ra, which is Ra ¼ aRw, where a is the Archie parameter and Rw is the resistivity of connate water. The heavy red line in Fig. 2A, which is averaged over 15 m, is the calculated resistivity of the water-saturated sediments (Ro), which is given by the following equation:
Ro ¼
aRw
fm
¼
Ra
f
1:6
¼
0:4 0:17d=400
f1:6
(1)
where d is the subbottom depth in meters. According to Eq. (1), the apparent resistivity decreases linearly with depth. It is assumed that Ro calculated with Eq. (1) serves as an accurate baseline for the resistivity data at the KC 151-2 well. In estimating gas hydrate saturations, resistivities higher than the Ro baseline resistivities are assumed to be due to gas hydrate. The gas hydrate saturations estimated from the resistivity using Archie’s equation with m ¼ 1.6 and Ra as given by Eq. (1) are shown in Fig. 3. Although, nuclear magnetic resonance (NMR) porosities were measured at the KC 151-2 well, these porosities were not used in estimating gas hydrate saturations in this study because the formation is dominantly silty clays. In the presence of clay bound water with fast relaxation times, the apparent NMR porosities yield overestimates of gas hydrate saturations (Collett et al., 2003). When the pore space is filled with free gas instead of gas hydrate, the saturations calculated from the resistivity assuming gas hydrate pore fill are slightly different from that calculated assuming free gas owing to the density difference between gas and gas hydrate. Lee (2005) derived an approximation formula for
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A
B
Fig. 2. A) Measured resistivity with depth at the Keathley Canyon 151-2 well is shown as a thin line. The thick line is the baseline resistivity calculated using m ¼ 1.6 with Ra ¼ 0:4 0:17d=400 Um. The inset shows the detailed resistivity near the presumed BGHS or the location of bottom-simulating reflector (BSR). (B) Calculated resistivity of connate water (Rw) at Keathley Canyon 151-3 well using the Arps (1953) formula with measured temperature and salinity (dots) and estimated Rw at the Keathley Canyon 151-2 well from Ra as given in Eq. (1) with m ¼ 1.6 and a ¼ 1.6. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
estimating free-gas saturations from the saturations estimated by assuming gas hydrate pore fill.
2.3. Predicting P-wave velocity from resistivity and porosity logs When P-wave velocity measurements are lacking, P-wave velocities are often predicted from resistivity logs by using a leastsquares method (Rudman et al., 1975). However, as shown in Lee (1999, 2005), predicting velocities by explicitly inserting gas
hydrate saturations estimated from the resistivity into the acoustic equation is a more accurate method. To predict velocities from the resistivity data in this study, the BGTL parameter l must be known. This parameter can be estimated if porosity and velocity of water-saturated sediments (or baseline velocity) are known. Because there was no porosity log at the KC 151-3 well, porosities derived from a least-squares fit to the density porosity measured at the KC 151-2 well were used to estimate the BGTL parameter l ¼ 1.5. A brief description of the BGTL is given in Appendix A. The method of predicting velocities from the resistivity is: 1. Estimate gas hydrate saturations from the resistivity and porosity logs with m ¼ 1.6 in the Archie equation and apparent resistivity described previously (Eq. 1). 2. Using the gas hydrate saturations, porosity, and clay volume content, calculate P-wave and S-wave velocities from the BGTL (Lee, 2002) with l ¼ 1.5 for sediments above the base of gas hydrate stability (BGHS) 3. To compute velocities for sediments below the BGHS, use the Biot–Gassmann theory (BGT) with parameters calculated from the BGTL with free-gas saturations as shown in Lee (2004).
Fig. 3. Gas hydrate saturation (smoothed over 21 points or 3 m) estimated from the resistivity log using m ¼ 1.6 with Ra ¼ 0:4 0:17d=400 Um.
Fig. 4A shows P-wave and S-wave velocities predicted from resistivity and density logs with gas hydrate saturations estimated from the resistivity log (shown in Fig. 3) using the BGTL under the assumption that there is no free gas. If it is assumed that the location of the GHPB (or BSR) is at 392 mbsf, the free-gas saturations below the BSR are slightly less than those shown in Fig. 3 and the predicted P-wave velocity assuming free gas below 392 mbsf is shown in Fig. 4B. The P-wave velocities for partially gas-saturated sediments (PGSS) are calculated at logging frequency of w10 kHz using the White (1975) theory. Because of the velocity dispersion for the PGSS, the P-wave velocities at seismic frequencies below 100 Hz are much smaller than those shown in Fig. 4B.
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A
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B
Fig. 4. Resistivity-derived velocities at the Keathley Canyon 151-2 well by explicitly inserting the gas hydrate saturation into the modified Biot–Gassmann theory (BGTL) with l ¼ 1.5, volume clay content calculated from the gamma log, and differential pressure with depth under the normal pressure regime. (A) P-wave and S-wave velocities assuming all saturants are gas hydrate. (B) P-wave velocities assuming that free gas is present below 392 mbsf.
3. Synthetic seismogram In order to relate the resistivity-derived velocity log (Fig. 4) to seismic profiles, a synthetic vertical seismic profile (VSP) for the KC 151-2 well is generated and shown in Fig. 5. In generating the synthetic VSP, it is assumed that there is no BGHS at the depth 392 mbsf. Because the resistivity responses from free gas and gas hydrate are the same, it is not possible to infer free gas below the BGHS solely on the basis of the resistivity log. If it is assumed that there is a BSR at depth 392 mbsf, the synthetic VSP accounting for the free gas below BSR is almost identical to the one shown in Fig. 5 except for a larger amplitude at the BSR level. This is because the P-wave velocity from the PGSS at seismic frequencies is much smaller than that for GHBS at the logging frequency. A regional unconformity at w100 mbsf is represented by a strong peak in the synthetic. Two large peak–trough combinations clearly indicate the zones of high gas hydrate saturated intervals, and the assumed BSR is indicated as a reverse-polarity reflection.
1680 m s1, whereas the average velocity at the KC 151-2 well is 1790 m s1. The average velocity of the VSP data in this interval is 1740 m s1. Therefore, the average velocity of the measured velocity at the KC 151-3 well is about 4% smaller than the average velocity of
4. Discussion 4.1. Predicted and measured velocities The P-wave velocities measured at the KC 151-3 well using the DSI tool are shown in Fig. 6 along with the interval velocities derived from the zero-offset VSP data. The well log P-wave velocities at the KC 151-3 well are w4% less than those from the VSP. The lower part of Fig. 6 shows the calculated velocities from the resistivity log at the KC 151-2 well. Distinct velocity markers observed at both wells are correlated with dotted lines. Note that the high velocity measured near 220 mbsf at the KC 151-3 well agrees well with the predicted velocity at the KC 151-2 well. However, high velocities near 290 mbsf at the KC 151-2 well are missing at the KC 151-3 well. Overall, the measured velocities at the KC 151-3 well are less than the predicted velocities at the KC 151-2 well. The average velocity at the KC 151-3 well (depth range from 123 to 331 mbsf) is
Fig. 5. Synthetic vertical seismic profile generated at the Keathley Canyon 151-2 well using the resistivity-derived P-wave velocities (Fig. 4) and measured density.
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Fig. 7. Multichannel seismic line KC 1 (Hutchinson et al., in press) crossing the Keathley Canyon 151-2 well, with a synthetic seismogram inserted.
Fig. 6. Comparison of P-wave velocities measured at the Keathley Canyon 151-3 well and derived from the resistivity logs at the Keathley Canyon 151-2 well.
VSP data, whereas the average velocity predicted at the KC 151-2 well is about 3% higher. The difference between measured and predicted velocities is significant considering that the distance between the two wells is small (w10 m) and that the geology appears to be uniform on the basis of seismic data. The difference may be caused by intra-site variability, quality of well logs, or the presence of free gas. The gas hydrate occurrence appears to be controlled by fractures (possibly through-going faults) as seen on the resistivity-at-bit images from LWD data in KC 151-2 well. It is not uncommon to have correlation problems between wells. The resistivity log was acquired while drilling (LWD log), and the depth penetration is deeper than velocity or density logs. The adverse effects such as borehole washouts could have been minimal for the LWD logs. Therefore, the predicted velocities could be more accurate than the measured velocities. Also measured P-wave log velocities may be affected by free gas associated with the dissociation of gas hydrate. Because of the inaccurate S-wave velocities, it is not possible to evaluate the presence of free gas with the wireline velocity logs. However, it is speculated that the average P-wave velocity being lower than the VSP-derived velocity could reflect the impact of a small amount of free gas on velocity, as demonstrated for the velocity logs at the Hydrate Ridge, offshore Oregon (Lee and Collett, 2005). 4.2. Synthetic seismogram and 3-D seismic A seismic line crossing the KC 153-2 well site is shown in Fig. 7 with a synthetic seismogram generated from the resistivity-derived velocity log (the top part of synthetic VSP shown in Fig. 5). The
match between the synthetic seismogram and seismic data is fairly good. Note that the interpreted BSR on the seismic section matches the assumed BSR reflection shown in the synthetic. A good match of BSR reflections between seismic and synthetic seismogram provides confidence in the predicted P-wave velocity and the gas hydrate saturations estimated from the resistivity. The seismic response of the zone with high gas hydrate saturations near the depth 225 m (depth between 220 and 230 mbsf and designated the ‘‘upper zone’’) is indicated as strong peaks both in the synthetic and 3-D seismic data. The average gas hydrate saturation in this interval is w25%. This reflection amplitude decreases as the reflection event moves in the down-dip direction. However, the synthetic response for the zone with high gas hydrate saturations near the depth 275 m (between 264 and 298 mbsf, with four highly saturated intervals and designated the ‘‘lower zone’’) is much stronger than that in the seismic data. The average gas hydrate saturation for the lower zone is w24%. The measured velocity at the KC 151-3 well for the lower zone is much lower than the predicted velocity (Fig. 6) and predicts much lower amplitudes similar to those shown in the 3-D seismic data. The VSP data acquired at the KC 151-3 well also indicate weak reflections for this interval. The discrepancy between the two wells and between synthetic and surface seismic data is likely due to a localized small anomalous zone at the KC 151-2 well, an anomaly too small to be detectable in the surface seismic or VSP data.
4.3. Nature of the BSR at Keathley Canyon 151-2 Gas hydrate and free gas are both electrical insulators, so there is no difference in the response to the formation resistivity. Therefore, locating the BSR based only on the resistivity without knowing P-wave velocity entails uncertainty. The insert in Fig. 2A shows detailed resistivities near the assumed BSR. The shaded region is a high resistivity zone compared to the adjacent depth intervals, and the BGHS can be anywhere inside the shaded area at the KC 151-2 well site.
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The resistivity logs acquired at the Hole 994D and 995B during Ocean Drilling Program (ODP) Leg 164 at the Blake Outer Ridge, southeastern U.S., indicate sharp decreases of resistivities at the BGHS (Fig. 5 of Collett and Ladd (2000)). At these sites, the resistivities at and below BGHS follow the baseline resistivities fairly well. If the resistivity characteristic of BGHS at the KC 153-2 is similar to that at Blake Outer Ridge, the BGHS is possibly near 392 mbsf. The average free-gas saturation between 392 and 400 mbsf is w3%. The average gas hydrate saturation of high resistivity zone between 386 and 392 mbsf is w15% with an average velocity of 1940 m s1. Therefore, the BSR at the KC 152-3 well site appears to be caused by the BGHS with about 15% saturation overlying partially gas-saturated sediments with about 3% saturation.
4.4. Gas hydrate saturations and gas hydrate evidence Although the gas hydrate saturations estimated from the resistivity log indicate that saturations approach as much as 40% in several thin intervals (Fig. 3), Claypool (2006) indicated that the presence of gas hydrate was not fully verified in the KC 151-2 well except for indirect evidence of the gas hydrate such as cold spots, moussey sediment texture, and anomalously low pore water salinity (Kastner et al., 2008). Also, several recovered pressure cores indicated gas concentrations exceeding normal solubility, although no gas hydrate was visually identified (Claypool, 2006). During ODP Leg 164, gas hydrates were recovered as nodules and veins at the Blake Ridge. The well logs at the Blake Ridge indicate that the maximum gas hydrate saturation is w20% of the pore volume (Collett and Ladd, 2000), which is equivalent to w10% bulk volume of the sediment. An average porosity in the upper zone at the KC 151-2 well is w40%, so w16% of bulk volume is gas hydrate. Although the bulk volume of gas hydrate at the KC 151-2 is greater than that at the Blake Ridge (but water depth and BGHS are much deeper at the Blake Ridge than at the KC 151-2 area), gas hydrate was not recovered at the KC 151-2 well site. One interpretation is that gas hydrate saturation is overestimated at the KC 151-2 well and that there is not much gas hydrate. This means that the parameters for the Archie equation are incorrect; either the apparent resistivity is too low or the Archie cementation parameter is too small. However, gas hydrate saturations estimated from the resistivity log using the Humble equation (Winsauer et al., 1952) are w30% for high gas hydrate saturated intervals. This still yields an estimate of w12% bulk volume of the sediment being occupied by gas hydrate at KC 151-2. The resistivity of connate water can be calculated using the Arps formula (Arps, 1953) with salinity and temperature. The Arps formula is Rw2 ¼ Rw1 ðT1 þ 7Þ=ðT2 þ 7Þ, where Rw1 and Rw2 are water resistivities at Fahrenheit temperatures at T1 and T2, matched by laboratory measurement and subsurface formation conditions, respectively. The calculated Rw (dots in Fig. 2b) is w0.275 Um near the seabed and is w0.15 Um at the subbottom depth of 400 m when using the seabed temperature of 4.79 C, a thermal gradient of 30 mK m1 (Hutchinson et al., in press), and pore water salinity measured from the core (Kastner et al., 2008). The estimated Rw (solid line in Fig. 2b) from the Ra in Eq. (1), with a ¼ 1.6, is Rw ¼ 0.25 Um at the seabed and Rw ¼ 0.14 Um at d ¼ 400 m. Except at shallow depths (less than 50 mbsf), estimated resistivities of connate water from the Archie analysis agree well with the Rw calculated using the Arps formula. Therefore, unless there are significant errors in the measurements, the gas hydrate saturations estimated from the resistivity log seem to be reasonable. In other words, the high gas hydrate saturations are not likely due to the error in the data analysis.
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What are other possible causes of not physically observing the gas hydrate in cores at intervals with high resistivities, particularly at the upper zone (220–230 mbsf)? The reasons include: (1) High resistivity caused by low porosity. Archie’s equation indicates that the formation resistivity of water-saturated sediment is greater than 3 Um if porosity is less than about 10% when using m ¼ 1 with Ra ¼ 0.45 Um or less than 20% when using m ¼ 1 and Ra ¼ 0.55 Um. However, the average porosity for this interval is w41% and not much different from that of adjacent intervals with no or only small degrees of gas hydrate saturation. Therefore, the argument that low porosity may be responsible for the log measured high resistivities is ruled out. (2) High resistivity is caused by low salinity. Salinity of connate water significantly affects the measured formation resistivity. If the pore water is fresh in this interval, the resistivity of the formation would be high without any gas or gas hydrate in the pore. The pore water analysis of cores in this interval indicates elevated pore water salinities exceeding 50 ppt. Using the Arps formula with an in situ temperature of 10 C and salinity of 50 ppt, the resistivity of pore water is w0.2 Um. The calculated formation resistivity of this interval assuming water saturation is w0.85 Um when using the Humble equation with calculated Rw (or 0.3 Um if we use m ¼ 1), and this value is much less than the observed anomalous resistivity, which is greater than 2–3 Um. To have greater than 2 Um resistivity, the salinity of pore water should be much less than the measured salinity. Therefore, variations in pore water salinity cannot explain the high resistivities in this interval. (3) High resistivity is caused by free gas instead of gas hydrate. Even inside the gas hydrate stability zone, free gas can co-exist with gas hydrate (Guerin et al., 1999; Lee and Collett, 2005). So it can be assumed that this interval contains free gas. If this is the case, the P-wave velocities should be much less than those shown in Fig. 4, and the BSR reflection from the synthetic would not agree with the BSR interpreted from the 3-D seismic data. Also, the measured velocity at the nearby KC 151-3 well indicates high velocities in this interval. Therefore, free gas may not be a possible primary cause of high resistivity of this interval. (4) The morphology of gas hydrate in the pore space and extended time of core recovery may be responsible. If the gas hydrate is disseminated in the pore space without forming large nodules or veins, the gas hydrate in the core may be easily dissociated during the long trip to the surface. Consequently, only indirect evidence for the presence of gas hydrate (e.g., cold spots or moussey sediment texture) would remain in the core. From an analysis of cores at the Hydrate Ridge offshore Oregon, Milkov et al. (2004) noted that the detection of finely disseminated gas hydrate in deep marine sediments depends mainly on indirect evidence because of rapid decomposition of the gas hydrate upon core recovery, and this observation supports our speculation. However, in order to substantiate this speculation, numerical modeling of dissociation of gas hydrate due to changing temperature and pressure of core during the recovery should be performed. 5. Conclusions The LWD resistivity log at the KC 151-2 well enabled us to predict accurate P-wave velocities and to generate a synthetic seismogram which agrees well with a 3-D seismic line. A faint BSR in this area is possibly caused by gas hydrate-bearing sediments with w15% saturation overlying partially gas-saturated sediments with very low saturations of w3%. The analysis of resistivity log
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indicates that the gas hydrate saturations reach w30–40% of the pore space within two zones: an upper zone (220–230 mbsf) and lower zone (264–298 mbsf). The amplitude of the synthetic seismogram for the upper zone agrees well with the amplitude in the 3-D seismic data. But the amplitude of lower zone does not agree with 3-D seismic data, implying that the lower interval may be a small localized anomalous zone and not continuous across the site. The absence of physically observed gas hydrate in cores with high gas hydrate saturations estimated from the resistivity log bring us to an enigma in interpreting both cores and logs. It is speculated that the gas hydrate morphology, which is finely disseminated in the pore space or in the fractures without large nodules or veins, in combination with poor core recovery, is responsible for the absence of observed gas hydrate in the cores.
Appendix A
G ¼ 0:9552 þ 0:0448 eCv =0:06714 0:18S1=2
(A6)
where p is differential pressure in MPa, l is a constant related to the degree of consolidation, Cv is the decimal clay volume content of the grains, and S is the decimal gas hydrate saturation in the pore space. For a detailed discussion of parameters n and G, consult Lee (2002). The parameter l is given by the following form when the rate of porosity change with respect to differential pressure is known (Lee, 2002):
l ¼ 1:0 þ 4:953 e5212vf=vp
(A7)
where p is in MPa. For unconsolidated sediments, the rate of porosity change with respect to differential is large (large negative number) and l becomes small. Generally l for the unconsolidated sediments varies between 1 and 3 and is treated as a free parameter to fit the measurements.
Modified Biot–Gassmann theory (BGTL) References The essence of the BGTL is to derive the bulk modulus of the sediments from the Biot–Gassmann theory (BGT) and shear modulus of the sediment from the velocity ratio in the form of Vs ¼ Vp Gað1 fÞn , where Vp is the P-wave velocity, Vs is the S-wave velocity, ais the Vs/Vp of the grains, and fis the porosity. G and n are parameters depending on differential pressure, consolidation, and clay content. The bulk modulus of sediments (k) is given by the following equation (the Biot–Gassmann theory): 2
k ¼ kma ð1 bÞ þ b M
(A1)
where
1 ðb fÞ f þ ¼ kfl M kma and the shear modulus is given by the following equation (BGTL formulation):
mma G2 ð1 fÞ2n k h i. kma þ 4mma 1 G2 ð1 fÞ2n 3
m ¼
(A2)
In Eqs. (A1) and (A2), kma,mma ,kfl ,f, and b are the bulk modulus of grains, the shear modulus of the grains, the bulk modulus of the fluid, porosity, and the Biot coefficient, respectively. Elastic velocities of water-saturated sediments can be computed from the elastic moduli by the following formulas:
Vp ¼
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi k þ 4m=3
r
and
Vs ¼
rffiffiffi
m r
(A3)
where r is density of the formation given by r ¼ ð1 fÞrma þ fr,and rma and rfl are the grain density and pore fluid density, respectively. For soft rocks or unconsolidated sediments, the following Biot coefficient is used (Lee, 2002):
b¼
184:05 þ 0:99494 1 þ expfðf þ 0:56468Þ=0:10817g
(A4)
The exponent n is given by
h n ¼ 10ð0:4260:235 log10 and G is given by
pÞ
i. l
(A5)
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