Physical-properties-of-the-shallow-sediments-in-late-Pleistocene-formations-Ursa-Basin-Gulf-of-Mexic

Physical properties of the shallow sediments in late Pleistocene formations,Ursa Basin,Gulf of Mexico,and their implications for generation and preservation of shallow overpressures
N.T.T.Binh a ,*,T.Tokunaga a ,1,T.Nakamura b ,2,K.Kozumi b ,2,M.Nakajima b ,2,M.Kubota b ,2,H.Kameya c ,3,M.Taniue c ,3
a
Department of Environment Systems,School of Frontier Sciences,University of Tokyo,5-1-5Kashiwanoha,Kashiwa-shi,Chiba 277-8563,Japan b
Geotechnical Department,Dia Consultants,Saitama,Japan c
Core Laboratory,Oyo Corporation,Niigata,Japan
a r t i c l e i n f o
Article history:
Received 14September 2007Received in revised form 20January 2009
Accepted 23January 2009
Available online 31January 2009Keywords:
Basin modelling
Shallow overpressure Fluid flow
Deepwater environment
a b s t r a c t
Understanding the evolution of abnormally high fluid pressures within sedimentary formations is critical for analysing hydrogeological processes and assessing drilling risks.We have constructed a two-dimensional basin model and have performed numerical simulations to increase the understanding of the history of fluid flow and shallow overpressures in the Pleistocene and Holocene formations in the Ursa basin,deepwater Gulf of Mexico.We measured physical properties of sediments,such as porosity and permeability,in the laboratory and estimated in situ pore pressures from preconsolidation pressures.We obtained porosity–effective stress relationships from measurements of bulk density,grain density and preconsolidation pressures in the laboratory.Porosity–effective stress relationships were also obtained from downhole density logs and measured pore pressures.The porosity–effective stress and porosity–permeability relationships obtained were applied in two-dimensional basin simulations.Results showed that high pore pressures developed shortly after sediment deposition.Peaks in pore pressure ratios were related to high sedimentation rates of mass transport deposits and the incision of the Ursa teral flows from the area where the overburden is thick towards the area where it is thin have occurred at least since 30ka.Present pore pressure and temperature distributions suggest that lateral flows play a role in re-distributing heat in the basin.
Ó2009Elsevier Ltd.All rights reserved.
1.Introduction
Understanding pore pressure regimes and fluid flow patterns in sedimentary basins and their temporal changes is important for investigating the evolution of sedimentary basins,the stability of slopes,and related geodynamics.For example,lateral fluid flows in a sedimentary formation are controlled by pore pressure gradients,sedimentation rates,and permeability distribution (Bethke,1986).Flemings et al.(2005)speculated that focusing of fluid flows may result in slope instability on continental slopes.In 2005,the Inte-grated Ocean Drilling Program (IODP)conducted Expedition 308to
study shallow overpressure and fluid flow in the Ursa region,continental slope of the Gulf of Mexico.Eight holes were drilled at three well sites U1324,U1323and U1322.The wells were logged and in situ measurements were made.Geopressured sediments from Pleistocene and Holocene formations were found in these wells (Expedition 308Scientists,2005;Myers et al.,2007).
In this area,Byrd et al.(1996),Eaton (1999),Ostermeier et al.(2002),and Flemings et al.(2005)described the existence of shallow water flow phenomena,and discussed the problems encountered with shallow water flows.Shallow water flows are associated with a variety of drilling problems and seafloor damage,including uncontrolled flow of sands in the annulus of the borehole being drilled,borehole washouts,craters,and cracks on the seafloor (Alberty et al.,1997;Alberty,2000;Faul et al.,2000;Ostermeier et al.,2002).They are considered to be the cause of major problems for the oil and gas industry working in the Mark-Ursa region (Alberty,2000).The loss of many wells in the Ursa Development Project in the Mississippi Canyon Block 810was an extreme example of severe damage due to violent shallow water flows (Furlow,1998).
*Corresponding author.Department of Earth Sciences,Durham University,Durham DH13LE,United Kingdom.Tel.:þ44(0)1913343972;fax:þ44(0)1913342301.
E-mail address:binh.nguyen@ (N.T.T.Binh).1
Tel.:þ81471364708;fax:þ81471364709.2
Tel.:þ81486543011;fax:þ81486543833.3
Tel.:þ81252745656;fax:þ8125271
6765.Contents lists available at ScienceDirect
Marine and Petroleum Geology
journal homepage :www.else /locate /m
arpetgeo
0264-8172/$–see front matter Ó2009Elsevier Ltd.All rights reserved.doi:10.1016/j.marpetgeo.2009.01.018
Marine and Petroleum Geology 26(2009)474–486
Recently,Sawyer et al.(2007a)described the lithology of the main depositional elements in the Mars-Ursa region and interpreted the geological evolution of the area for the past70ky based on high resolution3D seismic data and well log data.Long et al.(2007)and Flemings et al.(2006,2008)presented pore pressure penetrometer measurements made during IODP Expedition308and documented vertical and lateral variation in overpressure at Sites U1322and U1324.They showed that overpressures have reached60%of the hydrostatic effective stress at Site U1322and70%at Site U1324. Sawyer et al.(2007b)and Dugan et al.(2007)integrated physical properties of mass transport complexes(MTCs)and concluded that consolidation behaviour in MTCs is different from that in the sedi-ments encasing the MTCs.Furthermore,low permeability in MTCs precludes drainage of overpressure in the Ursa region.
In this study,we conducted numerical modelling,supported by data from laboratory experiments using samples obtained from IODP Expedition308,to examine lateralfluidflow and pore pressure evolution in the Pleistocene and Holocene sedimentary sections of the Ursa basin.In the geotechnical laboratory,we measured physical properties of sediments such as porosity and permeability and estimated in situ pore pressures from preconsolidation pressures. Then porosity–effective stress relationships were constructed from IODP results and the estimated in situ pore pressures.The porosity–effective stress and porosity–permeability relationships obtained were applied in two-dimensional basin simulations.
Two-dimensional modelling cases presented here include the effects of both the uneven sedimentation rates along the cross-section and the ancient channel activities on the hydrogeological system.The modelling results can help to improve the under-standing of the history offluidflow and overpressure in the study area.
2.Geological setting
In this section,we summarize what is known from previous research results about the lithology and deposition of sediments in the Ursa basin.This information will be used to construct the geological model for our basin modelling study which will be dis-cussed in Section4.
The Ursa Basin is located about200km southeast of New Orleans on the continental slope of the Gulf of Mexico(Fig.1).It is a salt-withdrawal mini-basin with water depth in the range800–1400m,and with sediments originating from the Mississippi River system(Expedition308Scientists,2005).This study focuses on the Late Pleistocene to Holocene sedimentary section of the Ursa Basin, i.e.,from the base of the Blue Unit to the seafloor(Fig.2).
According to Sawyer et al.(2007a)and Expedition308Scientists (2005),sediments deposited from the base of the Blue Unit to the seafloor can be divided intofive units from bottom to top:the Blue Unit,the Ursa Canyon channel-levee system,the Southwest Pass Canyon channel-levee system,mass transport deposits,and the distal deposits and hemipelagic drape(Fig.2).The Blue Unit is composed of interbedded sands and clays with a maximum two-way travel time of250–300ms(Sawyer et al.,2007a).In the study area,the Blue Unit thins towards west due to the incision by the Southwest Pass and Ursa Canyon systems(Fig.2)(Sawyer et al., 2007a).The Blue Unit was most likely deposited during the MIS Stage4eustatic sea level fall and at the time of an eastward shift in the drainage pattern of the Mississippi River(Piggott and Pulham, 1993;Winker and Booth,2000;Ruddiman,2001).The base of the Blue Unit was interpreted to have been deposited at around68ka based on biostratigraphic data from nannofossils assemblages (Winker and Booth,2000).Note that Urgeles et al.(2007)recently suggested the age of the base of the Blue Unit to be89ka,but without giving a reason.Thus,we assumed the age of deposition to be68ka. The Ursa Canyon runs from the northwest to the southeast(Sawyer et al.,2007a).The channel-levee system is composed of a channelfill, channel-margin slides,and levees(Sawyer et al.,2007a).The channel margin and levees consist of silty clays while the channelfill consists of sands and silty clays(Sawyer et al.,2007a).The Ursa Canyon incised the Blue Unit and channel-margin slides play as hydraulic barrier within the Blue Unit(Fig.2)(Sawyer et al.,2007a
).
Fig.1.Location of the study area and the cross-section used for two-dimensional basin modelling.Contours show water depth.The names of the blocks are also shown.After Sawyer et al.(2007b).
N.T.T.Binh et al./Marine and Petroleum Geology26(2009)474–486475
The Southwest Pass Canyon system is younger than,and lies to the west of the Ursa Canyon system (Fig.2).It eroded much of the western levee of the Ursa channel and completely buried its fill and eastern levee.It has similar characteristics as the Ursa Canyon system but is even larger (Sawyer et al.,2007a ).The Southwest Pass Canyon also contains a belt of rotated channel-margin slides (Fig.2),which is up to 5.5km wide.The canyon fill itself is approximately 1.3–1.6km wide (Sawyer et al.,2007a ).
Mass transport deposits (MTDs)are situated within the mud-rich levee deposits above the Blue Unit in the studied area (Fig.2)(Sawyer et al.,2007a ).MTDs consist of silty clays (Sawyer et al.,2007b;Dugan et al.,2007).The first mass transport deposit is located on the eastern levee of the Ursa Canyon channel-levee system,and the second mass transport deposit is located within the eastern levee of the Southwest Pass Canyon channel-levee system (Fig.2b)(Sawyer et al.,2007a ).
Distal deposits and hemipelagic drape overlie the MTDs.The distal deposits are composed primarily of olive-green and reddish brown clay interbedded with black clay and are capped by hemi-pelagic clay that is rich in nannofossils and foraminifera (Expedi-tion 308Scientists,2005).
3.Physical properties of the soft sediments
Soft sediment samples obtained from IODP Expedition 308were analysed,focusing mainly on porosity–permeability relationships.Index properties of the samples used for consolidation tests are shown in Table 1.Grain densities were in the range 2760–2900kg/m 3and were found to be higher than those measured onboard the ship (2667–2780kg/m 3)(Expedition 308Scientists,2005).We fol-lowed JIS A 1202-1999(Japanese Industrial Standards Committee,1999)and JGS 0111-2000(Japanese Geotechnical Society,2000a )for the measurements of the grain densities of the samples,so the reasons for the discrepancy between the density measurements are unknown.However,the effects of the differences of the grain densities on the calculated porosities were rather small,i.e.,less than 2%,and hence were not significant,at least for this study.3.1.Porosity–permeability relationships
3.1.1.Evaluation of permeability from 1-D consolidation tests
A total of fourteen samples were tested either by using an incremental loading oedometer (IL)or a
constant-rate-of-strain
Fig.2.(a)Seismic line AA 0and (b)interpreted cross-section.The dotted blue box shows the modelled area.Modified from Sawyer et al.(2007a)(For interpretation of the references to colour in this figure legend,the reader is referred to the web version of this article).
N.T.T.Binh et al./Marine and Petroleum Geology 26(2009)474–486
476
oedometer (CRS).Both the IL and CRS oedometers had a loading capability up to 10MPa.
Prior to testing,the samples were removed from the sealed core liner and trimmed to a height of 2.5cm and a diameter of 6.0cm.Once trimmed,the wet mass of the sample was recorded before the sample was transferred into the consolidation cell.The cells for both oedometers were made of stainless steel so that only vertical sediment deformation could occur.The sample was set into the cell and placed between two porous stones.Saturated filter papers were used to separate the sample from the porous stones and to prevent fine-grained sediment particles from blocking the drainage paths through the porous stones.After the cell was assembled,it was transferred to the oedometer.Side friction was minimized using silicone grease.General sample preparation and testing procedures followed the guidelines set out by JIS A 1217-2000and JIS A 1227-2000(Japanese Industrial Standards Committee,2000b,c )and JGS 0411-2000and JGS 0412-2000(Japanese Geotechnical Society,2000c,d ).
In the IL tests,the cells were submerged in saline water to keep the samples fully water-saturated.The samples were allowed to settle for 24h after the application of each incremental load.Time,loading pressure,and the change in specimen height were measured.The end of primary consolidation was evaluated using the square root of time method proposed by Taylor (1948).Then,the coefficient of vertical consolidation (C v )and the coefficient of volume compressibility (m v )were indirectly evaluated based on the one-dimensional Terzaghi’s consolidation theory (Terzaghi,1943;Lambe and Whitman,1979).
In the CRS tests,backpressure of 200kPa was applied to the samples for 24h to ensure they were fully water-saturated before loading with a constant axial strain rate of 0.03%/min.Backpressure was kept constant at 200kPa until the end of the tests.Time,axial load,the change in specimen height,and excess pore pressure at the base of the sample were measured.C v and m v were calculated using linear finite strain theory (Smith and Wahls,1969).
Hydraulic conductivity (K )was estimated from C v ,m v and density of water (r w )by
K ¼C v m v r w g
(1)
where g is gravitational acceleration (Lambe and Whitman,1979).In the equation (1),the unit of K is m/s,C v is m 2/s,m v is 1/Pa,g is m/s 2,r w is kg/m 3.
Intrinsic permeability can be obtained from hydraulic conduc-tivity through the following relationship (Hubbert,1940)
K ¼
k r w g
m
(2)
where k is intrinsic permeability (m 2)and m is viscosity of pore fluid (Pa s).The intrinsic permeability reported here is for seawater with density and viscosity to be 1025kg/m 3and 0.963Â10À3Pa s,respectively.
3.1.2.Results
The permeability values obtained from fourteen consolidation tests ranged from 5Â10À17to 1Â10À19m 2.For comparison,the results are plotted in Figs.3and 4with data from a previous study in the Gulf of Mexico (Bryant et al.,1975).Bryant et al.(1975)classified their measurements based on the grain size of the samples.For grain size analysis,we used a hydrometer and fol-lowed the method JIS A 1204-2000(Japanese Industrial Standards Committee,2000a )and JGS 0131-2000(Japanese Geotechnical Society,2000b ).We also estimated shale volume of the sediments from gamma ray log data.The analysed samples from MTDs and silty clays have shale volumes in the range 59–78%(Table 1).Thus,the data obtained for MTDs and silty clays were compared with group 2of Bryant et al.(1975),which had clay content in the range 60–80%(Fig.3).Since the gamma ray data were not good enough at very shallow depths near the seafloor,we did not use these data
to
Table 1
1E-19
1E-18
1E-17
1E-16
1E-15
Porosity
P e r m e a b i l i t y (m 2)
Silty clays MTDs Group 2 (Bryant et al., 1975)
Fig. 3.Porosity and permeability obtained from IL tests and CRS tests for mass transport deposits (open triangles)and silty clays (open squares)in the Ursa Basin,and the published results in the Gulf of Mexico (black circles)(Bryant et al.,1975).The dotted line and the black line were modelled porosity–vertical permeability relation-ship used for MTDs and for silty clays,respectively.
N.T.T.Binh et al./Marine and Petroleum Geology 26(2009)474–486
477
calculate shale volumes for hemipelagic clay.Based on visual core description and smear slide analyses,hemipelagic clay samples are mainly composed of clay (Expedition 308Scientists,2005).Therefore,we assume that the data for hemipelagic clays should be comparable with the group 1samples of Bryant et al.(1975)which had clay content higher than 80%(Fig.4).Based on these compar-isons,the results from this study were confirmed to be consistent with data from the previous study.
An IL test and a CRS test were conducted with specimens from the same sample U1324B-19H-CC for comparison.The intrinsic permeability values obtained from the CRS test were about 1.17–1.2times higher than those obtained from the IL test for the same porosity (Fig.5).However,the slopes of the permeability–porosity lines on the semi logarithmic plots were parallel (Fig.5),suggesting that the trend of permeability reduction by mechanical consoli-dation can be estimated from this slope.We used the equation
k ¼A exp ðB f Þ
(3)
where A and B are lithology-dependent constants to express rela-tionships between permeability (k )and porosity (4)(Table 2).We used the porosity–permeability relationships shown in Table 2for the two-dimensional simulation described in the next section.3.2.Preconsolidation pressure measurements
Preconsolidation pressure is usually interpreted to be the maximum effective overburden stress that sediments have expe-rienced (Casagrande,1936;Lambe and Whitman,1979).Knowing the preconsolidation pressure (P c )and in situ overburden stress (S v ),we can estimate the in situ pore pressure (P p )through the following equation assuming that the sediments are normally consolidated.
P p ¼S v ÀP c (4)
In this study,preconsolidation pressures of the sediments were determined by using Casagrande (1936)’s method to analyse the IL and CRS test data.
In situ pore pressures calculated from P c are shown in Table 3.The values obtained are consistent with in situ measurements (Flemings et al.,2008)(Fig.6)except for the point at 1566mbsl (metres below sea level)at site U1324.Also,in situ pore pressure
measurements at 1316.5mbsl and 1366.5mbsl seemed to be lower than the expected pore pressures from preconsolidation pressures.The in situ measurements were conducted with good coupling between the measurement device and the formation at these depths (Flemings et al.,2008).These differences could be explained by the declines of pore pressures due to recent drilling activity,as suggested at the site U1322by Flemings et al.(2008)even though the precise reason is quite difficult to state.
Based on the data shown in Fig.6,we consider that the sections between seafloor and 1127mbsl at site U1324and between seafloor and 1375.5mbsl at site U1322are normally consolidated.3.3.Porosity–effective stress relationships
Porosity–effective stress relationships were constructed for normal consolidation sections based on porosities calculated from density logs at sites U1324and U1322(Expedition 308Scientists,2005)(Fig.7).Overburden stress was estimated from density logs and water depths.
In hemipelagic clay sections,porosities decrease quickly from 80%to 55%(Fig.7(a)).For mass transport deposits and silty clays,porosities can be modelled by porosity–effective stress relation-ships of the Athy (1930)type,with different parameters for the silty clays and mass transport deposits (Fig.7(b),(c)).
4.Two-dimensional pore pressure and fluid flow modelling Basin modelling is one of the methods which can be used to predict pore pressure as well as fluid flow in a sedimentary basin
(e.g.,Welte and Yalcin,1988;Burrus et al.,1992;Dore
´et al.,1993;Hermanrud,1993;Neuzil,2003).Fully coupled and integrated basin simulators provide information on pore pressures,porosities,temperatures and fluid flow patterns through time.We conducted two-dimensional simulation using SIGMA-2D (Okui et al.,1996,1998).
1.E-19
1.E-18
1.E-17
1.E-16
Porosity
P e r m e a b i l i t y (m 2)
Fig.5.Porosity–permeability relationships obtained from the IL test (black squares)and
CRS test (open squares)on the sample 1324B-19H-CC.The lines were obtained by least square fits.
Table 2
Constants obtained for vertical permeability–porosity relationships.See text for 1.E-19
1.E-18
1.E-17
1.E-16
1.E-15
1.E-14
Porosity
P e r m e a b i l i t y (m 2)
Fig.4.Porosity and permeability obtained from hemipelagic clays in the Ursa Basin using IL consolidation tests (open squares),and the published results in the Gulf of Mexico (black circles)(Bryant et al.,1975).The black line was used as the modelled porosity–vertical permeability relationship for hemipelagic clays.
N.T.T.Binh et al./Marine and Petroleum Geology 26(2009)474–486
478
4.1.Model construction:geological model,lithology model,and physical properties
The cross-section was chosen to be parallel to the local slope direction in the study area (Fig.1).Hence,it is considered to contain the principal direction of fluid flow related to sedimentation and topography.There are three IODP Expedition 308well sites along this section,i.e.,U1324,U1323and U1322,and data from these wells can be used for calibration.
A lithology model was made based on previous studies in the area (Expedition 308Scientists,2005;Sawyer et al.,2007a ).Because of the lack of detailed data,it was necessary to simplify the lithology distribution.Four types of lithology,hemipelagic clays,mass transport deposits,sands,and silty clays,were used to model the sediments in the study area (Fig.8).Since sheet sands in the Blue Unit have been considered to be the source of shallow water flows in the Ursa area (Eaton,1999;Ostermeier et al.,2002;Winker and Shipp,2002),the distribution of sheet sands in the Blue Unit and the Ursa channel fill sands were modelled in detail,whereas the other lithologies were rather simplified.The modelled cross-section was part of cross-section AA 0in Fig.2(b),and was divided into 28columns,26layers and one erosion event (Fig.8).Based on previous research results (Expedition 308Scientists,2005;Sawyer et al.,2007a ),the distribution of the sediments was set as follows (Table 4).From seafloor to seismic horizon S10,the lithology is hemipelagic clays (layers 1and 2).From seismic horizon S10to seismic horizon S20,the lithology is silty clays and MTDs (layers 3–6).From seismic horizon S20to seismic horizon S30,the lithology is MTDs (layers 7–9).From seismic horizon S30to seismic horizon S60,the lithology is silty clays and MTDs with small amounts of sand (layers 10–13).The Ursa channel (layers 14–17)was modelled as sands and silty clays with the lithology distribu-tion being based on the seismic interpretation (Fig.2)(Sawyer et al.,2007a ).The Blue Unit includes sands and interbedded silty clays (layers 18to 24).Layers 25and 26are silty clays below the Blue Unit.The simulation was run for a total period of 85ky.
Erosion by the incision of the Ursa Canyon was considered (Fig.8).Based on previous studies on sea level cycles in the Mis-sissippi river depositional system and in the Gulf of Mexico (Ruddiman,2001;LoDico et al.,2006),we considered that erosion had occurred after the deposition of the Blue Unit and before the sea level rise at 58ka.The timing of erosion was set to lie between 60ka and 58ka (Table 4).The eroded thickness was modelled to be between 50and 250m,assuming that the thickness of the Blue Unit had been constant throughout this cross-section before erosion occurred.During erosion,the palaeo-water depth in the eroded area increased by an amount equals to the eroded thickness.
The relationships between porosity and vertical effective stress and between porosity and permeability for hemipelagic clays,MTDs and silty clays obtained from our experimental study (Figs.3,4and 7and Table 2)were used as input data for basin simulation.We also used published relationships between porosity and vertical effective stress and between porosity and permeability for sands in the Gulf of Mexico (Aniekwena et al.,2003)(Fig.9).
Anisotropy in permeability was considered.Freeze and Cherry (1979)suggested that clays and shales show horizontal to
vertical
Table 3
a
U1324
U1322
Pressure (MPa)D e p t h (m b s l )
Overburden stress
Calculated from Pc
Pressure (MPa)
Fig.6.Pore pressures calculated from preconsolidation pressures (filled squares)(a)at site U1324and (b)at site U1322.The in situ measurements (Flemings et al.,2008)are also shown by open triangles.The units mbsl are metres below sea level.
N.T.T.Binh et al./Marine and Petroleum Geology 26(2009)474–486
479
anisotropy ratio in the range3:1–10:1.In this study,the ratios between horizontal permeability and vertical permeability were chosen to be10:1for hemipelagic clays and mass transport deposits,and to be3:1for silty clays.For sands,since the perme-ability in the horizontal direction was found to be nearly equal to,or only slightly larger than that in the vertical direction(Hatanaka et al.,1997),the horizontal to vertical anisotropy ratio was chosen to be1:1.
Other physical properties for each lithology required for the two-dimensional basin modelling include grain density,matrix thermal conductivity,and heat capacity.These data were estimated from published results(Expedition308Scientists,2005;Rossane et al.,2004;Waples and Waples,2004)(Table5).4.2.Boundary and initial conditions
Geological information can be used to select the appropriate boundary conditions.For this purpose,the distribution of faults in the layers of higher hydraulic conductivity was studied.
Because the majority of sediments are clays and silty clays,and fault displacements tend to producefine-grained material in general(e.g.,Ferrill and Morris,2001),all faults are assumed to act as sealing faults.Faults are located at the eastern boundary of the cross-section(Fig.2(b)).At the western boundary,the Blue Unit is completely eroded by the incision of the Southwest Pass channel (Fig.2).Thus,the body of low hydraulic conductivity in the Southwest Pass channel can be treated as a seal for westward
fluid Fig.8.The geological model used for the basin modelling.Dotted line shows erosion by the Ursa Canyon.
50
100
150
200
250
Porosity
V
e
r
t
i
c
a
l
e
f
f
e
c
t
i
v
e
s
t
r
e
s
s
(
k
P
a
)
100
200
300
400
500
600
700
800
Porosity
V
e
r
t
i
c
a
l
e
f
f
e
c
t
i
v
e
s
t
r
e
s
s
(
k
P
a
)
Porosity
a b c
Fig.7.Porosity–effective stress relationships for(a)hemipelagic clays,(b)MTDs,and(c)silty clays.Lines in thefigure and the equations show the modelled porosity–effective stress relationships used for two-dimensional basin modelling.
N.T.T.Binh et al./Marine and Petroleum Geology26(2009)474–486
480
flow in the Blue Unit.Therefore,no-flow boundary conditions were assigned to both side boundaries.For the basal boundary,no fluid flow conditions were set because the base of the cross-section is composed of a mud-rich layer.
The initial pore pressures at each time step were set to be the calculated pore pressures at the previous time step.For the newly added grids,hydrostatic pore pressures were applied.The initial temperatures were set to be those obtained from the previous time step for existing grids,and seawater temperature for the newly added grids.Boundary conditions for the heat flow in SIGMA-2D are specified temperatures along the upper boundary,a specified heat flux along the bottom,and no flux along side boundaries (Okui et al.,1996,1998).At the upper surface of the model,temperature
and pressure were modelled as seafloor temperature and seafloor pressure.Temperatures at the seafloor were set to be in the range 4.0–4.5 C depending on the water depth.Pressure at the seafloor was considered to be hydrostatic.4.3.Model calibration
During calibration,the measured data from wells,i.e.,pore pressure,temperature,and porosity,were compared with the simulated results at the well locations to optimize the parameters used in the model.Sedimentation rates were adjusted by trial-and-error to obtain reasonable agreement between simulated and observed values.
The geological model used in this study was made up of 26layers (Fig.8and Table 4).In the modelled cross-section,the ages of the top of the layers 1,2,6,9,12,13,15and 19were determined based on the estimated ages of the known key horizons (Table 4)(Expedition 308Scientists,2005;Sawyer et al.,2007a ).Because other layers were situated between horizons of known ages,the ages of these layers were adjusted to lie within the constrained ranges.Through this procedure,the appropriate sedimentation rates (Table 4)were chosen to obtain good correlations between the calculated results and the measured data.
Present-day heat flow at the base of the model was estimated by fitting temperatures calculated from the model and in situ measurements (Expedition 308Scientists,2005).The heat flow value obtained was 36.3mW/m 2.This value is quite low in comparison with heat flow values seen in the continental crust,which ranges from 40to 46mW/m 2(Smith and Dees,1982).Mello and Karner (1996),Nagihara and Jones (2005)and Jones et al.(2003)suggested that the rapid deposition of thick sections of young sediments in the Mississippi fan has suppressed regional isotherms,resulting in anomalously low heat flow.We assumed that the palaeo heat flows were the same as present-day heat
flows.
Table 4
Ages and thicknesses for the modelled cross-section AA 0.S10to S80represents key
Table 5
Physical properties of four types of lithology estimated from Expedition 3080500
1000
150020002500300035004000
0.30.40.50.60.70.8
Porosity
V e r t i c a l e f f e c t i v e s t r e s s (k P a
)
1.0E-15
1.0E-14
1.0E-13
1.0E-12
1.0E-11
1.0E-10
0.1
0.20.30.40.5
Porosity
P e r m e a b i l i t y (m 2)
a
b
Fig.9.(a)Porosity–effective stress and (b)porosity–permeability relationships for sand.From Aniekwena et al.(2003).
N.T.T.Binh et al./Marine and Petroleum Geology 26(2009)474–486
481。