Horizontal-stress-contrast-in-the-shallow-marine-sediments-of-the-Gulf-of-Mexico-sites-Walker-Ridge

Horizontal stress contrast in the shallow marine sediments of the Gulf of Mexico sites Walker Ridge 313and Atwater Valley 13and 14e Geological observations,effects on wellbore stability,and implications for drillingRichard Birchwood *,Sheila NoethSchlumberger,1325South Dairy Ashford Road,Houston,TX 77077,USAa r t i c l e i n f oArticle history:Received 15August 2011Received in revised form 12January 2012Accepted 17January 2012Available online 25January 2012Keywords:Wellbore stability Geomechanics DrillingGas hydrateMethane hydrate AnisotropyHorizontal stress Backreaminga b s t r a c tSigni ficant horizontal stress anisotropy was encountered in three blocks in the Gulf of Mexico drilled by the Gulf of Mexico gas hydrates Joint Industry Project,namely,Atwater Valley 13,Atwater Valley 14,and Walker Ridge 313.The geological factors responsible for this state of stress and the implications for wellbore stability and drilling operations are explored.In Atwater Valley 13and 14,stresses at the well sites were perturbed by the underlying salt.In Walker Ridge 313,a combination of thrusting by adjacent salt and material anisotropy associated with the regional minibasin structure caused the maximum horizontal stress to rotate with depth.Horizontal stress anisotropy combined with a lack of heavy mud caused breakouts to form in the well Walker Ridge 313-G (WR313-G).A simple formula to predict the depth at which wells drilled with seawater are prone to breakouts is derived.Evidence is presented that the breakouts in WR313-G produced heavy cavings which were mainly responsible for the tight hole conditions encountered while drilling this well.However,it is shown that bottom hole assembly (BHA)design and drilling practices such as backreaming may have exacerbated these problems.Modi fications to drilling practices based on the experience gained from drilling WR313-G helped to improve the quality of the neighboring borehole WR313-H.Ó2012Elsevier Ltd.All rights reserved.1.IntroductionA frequently made assumption when modeling shallow marine sediments in the Gulf of Mexico is that the horizontal principal stresses are equal or close to equal (Ozkale,2006;Wojtanowicz et al.,2000).This paper discusses recent evidence of strong hori-zontal stress contrast from two drilling expeditions of the Gulf of Mexico Gas Hydrate Joint Industry Project (JIP).During the Leg I campaign in April and May of 2005,the JIP drilled and cored a series of riserless vertical wells at potential gas hydrate sites located at Atwater Valley and Keathley Canyon in the Gulf of Mexico.In Atwater Valley,two wells were drilled with conventional rotary bits (AT13#1and AT14#1)and three were drilled with corers (AT13#2,ATM #1,ATM #2).In Keathley Canyon,one well was drilled with a conventional rotary bit (KC151#2)and one was cored (KC151#3).The expedition was focused on drilling,logging,and coring operations for evaluation of gas hydrate related hazards associated with drilling through clay-dominatedsediments that are typical of the shallow sub-sea floor in the deepwater Gulf of Mexico (Ruppel et al.,2008).In 2009the Leg II expedition of the JIP aimed at evaluating gas-hydrate occurrences in coarser-grained sediments.The JIP conducted logging-while-drilling operations that included drilling and logging seven wells in three blocks:Green Canyon 955,Walker Ridge 313,and Alaminos Canyon 21(Boswell et al.,2012;Collett et al.,2011;Shedd et al.,2012).The locations of these blocks are shown in Figure 1.A constant challenge during the drilling campaign was to get optimal data quality by maintaining borehole stability in shallow unconsolidated sediments.According to Collett et al.(2009)many of the encountered drilling hazards were not gas hydrate-related.Issues such as borehole instability,drill cuttings removal,in fluxes of free gas and an apparent shallow water flow were among the problems faced during the two expeditions.In spite of these problems,most wells successfully reached their targets.Insights into optimizing drilling strategies for marine riserless drilling programs were gained.In this paper we analyze and review the stress field and stress directions as determined from features indicative of mechanical failure of the wellbore wall observed in logging-while-drilling images and caliper data from the Atwater Valley 13and 14sites*Corresponding author.E-mail address:rbirchwood@ (R.Birchwood).Contents lists available at SciVerse ScienceDirectMarine and Petroleum Geologyjournal ho mep age:www.elsevier.co m/lo cate/marpetgeo0264-8172/$e see front matter Ó2012Elsevier Ltd.All rights reserved.doi:10.1016/j.marpetgeo.2012.01.008Marine and Petroleum Geology 34(2012)186e 208(Leg I)and the Walker Ridge 313site (Leg II).Two types of failure modes are most commonly observed in wellbores,namely “breakouts ”,a form of compressive failure and “drilling-induced fractures ”,a form of tensile failure.Both types of failures can provide information about stress orientations.Drilling-induced compressive rock failure leading to breakouts occurs where the maximum hoop stress is higher than the rock strength (Zoback,2007).In a well drilled parallel to the vertical principal stress Sv,borehole breakouts tend to be aligned with the azimuth of the minimum horizontal stress.On the other hand,drilling-induced tensile fractures develop along the azimuth where the hoop stress is most likely to go into tension.These fractures tend to be aligned with the azimuth of the maximum horizontal stress.Both breakouts and fractures can be identi fied from resistivity images.Tensile fractures look like cracks along the wellbore wall,while breakouts usually cover a bigger area of the wellbore and the edges are not so well de fined.In vertical wells,both features tend to be aligned with the well axis and occur as conjugate pairs separated by 180 .Hence they can be distinguished from natural fractures or failure of the borehole wall caused by hydraulic erosion or mechanical wear.Since tensile fractures observable on wellbore images occur when the wellbore pressure exceeds the fracture gradient,this mode of failure can frequently be correlated with the existence of high wellbore pressure or ECD.1Zoback and Zoback (1980)used breakout information as well as earthquake focal mechanisms to build regional stress maps.Borehole breakouts as stress indicators are also described by Bell and Gough (1979),Plumb and Cox (1987),Plumb and Hickman (1985),Yassir and Zerwer (1997),and others.Examples ofdrilling-induced tensile fractures as stress indicators are shown in Castillo et al.(2000),Wiprut and Zoback (2000),and Zoback (2007).The Gulf of Mexico is considered to be a passive margin basin in a plate tectonic context with an extensional stress regime (Stein et al.,1989),but it is actively deforming because of the interplay of salt tectonics and sedimentary loading (Peel et al.,1995).The magnitude of the horizontal stresses in the Gulf of Mexico is not expected to be larger than that of the vertical stress.Rapid depo-sition at the shelf edge causes slumping,which results in some stress anisotropy.However,horizontal stresses near the seabed are sometimes assumed to be equal to the vertical stress due to their plastic nature (Wojtanowicz et al.,2000).In such circumstances,drilling-induced fractures and breakouts should not exhibit a preferred orientation.However in this paper,evidence of consistently oriented wellbore damage features in sediments located less than 300feet below the sea floor is presented for sites in Atwater Valley 13,Atwater Valley 14,and Walker Ridge 313.In the next section,the geological settings of the wells in Atwater Valley and Walker Ridge are described.In Section 3,image logs showing evidence of signi ficant horizontal stress contrast at both sites are presented.In Section 4,possible geological factors that could explain the observed horizontal stress anisotropy are considered.In Section 5,the implications of high horizontal stress contrast for wellbore stability and drilling operations are explored for the Walker Ridge sites.2.Geologic frameworkThe present-day Gulf of Mexico is characterized by a complex interaction of salt tectonics and sedimentary loading.The rifting during the Middle Jurassic created deep graben and elevated horst structures whose topography very probably in fluenced the post-rift deposition of the Upper Jurassic Louann Salt (Peel et al.,1995).Due to deposition of a high volume of Cretaceous e Cenozoic clastic sedi-ments,the shelf margin prograded several hundredkilometersFigure 1.Location map of the northern Gulf of Mexico showing the different JIP sites.JIP Leg I sites are displayed by green dots.The red stars indicate locations drilled during Leg II.Note:The site in AC818was not drilled since it revealed evidence of overpressures (figure modi fied from Boswell et al.,2009).(For interpretation of the references to color in this figure legend,the reader is referred to the web version of this article.)1ECD stands for “Equivalent Circulating Density ”.It is the density of a static column of drilling fluid required to generate the pressure at a given location in the borehole during mud circulation.The pressure developed during mud circulation is higher than the pressure exerted by a static mud column of the same density due to frictional effects.R.Birchwood,S.Noeth /Marine and Petroleum Geology 34(2012)186e 208187and the underlying Jurassic salt began to mobilize as diapirs and salt canopies driven by differential loading of the prograding clastic wedge.Salt-withdrawal minibasins withflanking salt bodies and ridges as well as slumps and debrisflows can occur as isolated systems or as components of salt-based detachment systems and are characteristic and very well documented(Diegel et al.,1995;Nelson, 1991;Prather et al.,1998).The Sigsbee salt canopy comprises more than100salt sheets and stocks that coalesced to cover more than 137,000km2on the lower continental slope of the northern Gulf of Mexico.The canopy is advancing along thrust systems with the leading edge of the canopy thrusts following the base of the Sigsbee Escarpment,a bathymetric step of about1250m(Hudec et al.,2009). Detachment faults soling into salt have created salt-cored foldbelts in sediments near the Sigsbee Escarpment,such as the Perdido Foldbelt in the western Gulf and the Mississippi Fan Foldbelt in the east(Fiduk et al.,1999;Hutchinson et al.,2008;Rowan et al.,2000). Salt-withdrawal minibasins in the shelf and upper slope are gener-allyfilled while those in the lower slope are still beingfilled(Winker and Booth,2000).The highly variable deposition rates in the minibasins depend on varyingfluvial input and slope debris.During the late Oligocene e Miocene the main depocenter moved eastwards to the present-day location of the Mississippi River.Fluid and gas migration pathways are usually located along the crests of the ridges, at the edges of the minibasins and near slope failures with faults that extend to the seafloor.These permeability pathways contribute to a seafloor morphology with hydrocarbon vents and pockmarks, chemosynthetic communities,authigenic carbonate outcrops, seafloor mud volcanoes and gas hydrate mounds(Lorensen et al., 2002).In the following sections,the drilled sites in Atwater Valley and in Walker Ridge are described in more detail.2.1.Atwater ValleyThe Atwater Valley area,with water depths of1300m,is located within the Mississippi Valley channel complex,with a thick sedi-mentary layer covering the underlying salt.The Pleistocene sedi-ments are complex and indicate multiple channel levee and slope fan systems.The massive deposits of the Pleistocene slope fan containing sands and shales show slumping and channeling. Subsequently,sands and shales were deposited in channel-levee systems(Dai et al.,2004).Mobile salt resulted in formation of the northwest-southeast trending fault system,which provides permeable pathways for gas migration and formation of gas hydrates.This fault system is possibly active today and enables migration to the series of mounds found in this area.2.2.Walker RidgeThe Walker Ridge313site is located in the tabular salt and mini-basin province of the Gulf of Mexico below about6500ft of water approximately90km north of the Sigsbee Escarpment in the Terrebonne mini-basin.The mini-basin is bounded to the south, east and west by large allochthonous salt bodies that can crop out locally at the seafloor(McConnell et al.,2009).Thus the basin architecture represents a closed low with a sand-delivery fairway (Frye et al.,2012).A result of uplift of the bounding ridges is a reversal of the depositional gradient and deposition of“ponded”sheet sands(Beaubouef and Friedmann,2000).According to Frye et al.(2012)recent uplift and extension have occurred at a north-south trending salt diapir at the northeast end of the salt wall, but the minibasin has also undergone recent compression, indicated by the morphologic expression of thrust faults in the seafloor(McConnell and Kendall,2002).The compression and fault morphologies indicate recent northwest-to-southeast salt move-ment on the northern side of the minibasin.3.Analysis of borehole images in Atwater Valley and Walker Ridge3.1.Introduction to resistivity imagesHigh-resolution electrical images of the near-wellbore region were analyzed in both the Atwater Valley and Walker Ridge wells. These images were produced while drilling by taking continuous 360 oriented measurements of downhole resistivity.The GVR6 tool(GeoVision*)has two transmitter coils and several electrodes that enable the tool to focus current at three different depths of investigation.The resulting GVR6button images have an azimuthal resolution of about6 (Bonner et al.,1996).The GVR6data are plotted as a planar,“unwrapped”,360 resistivity image of the borehole with depth.The image orientation is referenced to north. Orientation is measured by the magnetometers inside the tool. Horizontal features appear horizontal on the images,whereas planar,dipping features are sinusoidal in aspect.Since the tool is generally located close to the bit,measurements are made shortly after the bit passes a given depth.This minimizes the effect of hole degradation on the measurement.In water-based mud,borehole breakouts are identified on the GVR6image because of tool standoff from the elongated sides of the borehole ellipse which results in lower resistivity in the direction of the breakouts(Goldberg and Janik,2006;McNeill et al., 2004).Drilling-induced fractures are revealed as linear,low-resistivity features since they contain drillingfluid(Rezmer-Cooper et al.,2000).3.2.Resistivity images in Atwater ValleyFigure2shows resistivity images from AT13#1and AT14#1. Both wells exhibited pervasive drilling-induced fractures that are mostly oriented in an east-west direction.The fractures were caused by the ECD in the wellbore exceeding the fracture gradient. High ECD developed in these wells because boreholes were sealed off from the seafloor by either creep or a solids influx accompanying a shallow waterflow.A more detailed description of these events can be found in Birchwood et al.(2007).The consistent orientation of the hydraulic fractures in these boreholes suggests that there is a strong horizontal stress contrast in the Atwater Valley area.Although the hydraulic fractures in AT13#1and AT14#1were directed mostly east-west,this was not uniformly the case.Figure3 shows a close-up view of the hydraulic fracture in the shallow section of AT13#1.Between4400and4410ft RKB2(123e133fbsf), the hydraulic fracture appears to be directed northeast-southwest.A change in the direction of the fracture occurs between4410and 4412ft RKB(133e135fbsf).For the rest of the logged interval the fracture is consistently oriented in an east e west direction.The apparent change in orientation is indicative of a change in the stress direction at this depth and may have been caused by the presence of a fault(see Section4.1).3.3.Resistivity images in Walker RidgeThe wells drilled in Walker Ridge(WR313-G and WR313-H)are located at the edge of a salt withdrawal basin,about800m apart. *Mark of Schlumberger.2RKB is short for rotary kelly bushing.The RKB is a steel housing located on a drilling platform used to transmit torque from the rotary table to the drillstring. The RKB constitutes a standard reference for measuring depth in a borehole.In AT13#1the RKB was located4277ft above the seafloor.In AT14#1,WR313-G, WR313-H the corresponding distances were4309,6614,and6501ft.All logs shown in this paper with MD in the depth track are referenced to RKB.R.Birchwood,S.Noeth/Marine and Petroleum Geology34(2012)186e208 188Breakouts were observed in both boreholes.However,the break-outs in WR313-G were far more pervasive than those in WR313-H.Through examination of breakouts and a drilling-induced fracture in WR313-G,subtle changes in stress direction can be observed.Figure 4shows two sets of drilling-induced fractures in the WR313-G well.These fractures constituted the shallowest indica-tors of the stress direction in this well and are oriented northwest e southeast.This suggests a maximum horizontal stress with the same orientation at this depth.However this stress direction appears to rotate clockwise with increasing depth.Figure 5shows a set of breakouts in a deeper section of the well between 7100and 7200ft RKB.These are oriented east e west suggesting that the maximum horizontal stress is directed north e south.However,breakouts occurring in the deepest sections of the well tended to be oriented west-northwest e east-southeast suggesting that the maximum horizontal stress was oriented north-northeast e south-southwest.Figure 6shows an example of these deeper breakouts.Note that in contrast to the breakouts shown in Figure 5,the breakouts in Figure 6appear as resistive rather than conductive features.The paradox of resistive breakouts was described by Cook (2010)who suggested that gas hydrate may have formed inside these breakouts.Evidence supporting Cook ’s hypothesis will be presented shortly.The stress indicators shown in Figures 4e 6suggest that the maximum horizontal stress rotates from northwest e southeast at the surface to north-northeast e south-southwest deeper in the well.The breakouts in WR313-H are identi fied only for depths between 9165ft and 9175ft RKB.Over this interval (Fig.7)they show a northwest-southeast direction,indicating a maximum horizontal stress direction of northeast-southwest.This is in general agreement with stress direction of N40 E stated by Cook (2010)based on gas hydrate filled fractures in WR 313H.As previously stated,many of the breakouts observed in WR313-G were strikingly resistive relative to the surrounding formation (Fig.6).This suggests that gas hydrates were present at the location of these breakouts.Two possible explanations can be cited.The first is that the wellbore penetrated gas hydrate zones and that for some reason,gas hydrate was preserved only at the location of the breakouts.However as Cook (2010)pointed out,logging-while-drilling measurements indicated that wellbore temperatures were generally maintained well below the methane hydrate phase transition temperature.It is therefore unlikely that drilling would have caused gas hydrates to preferentially dissociate outside the breakouts.The second possible explanation is that gas hydrates formed within the breakouts during drilling.Support for this explanation can be seen in Figure 8which provides evidence that the gas hydrates in breakouts were localized borehole features that were distinct from the formations that were drilled.The figure shows some of the shallowest resistive breakouts recorded in the hole.Two resistivity logs,one a deep-reading 40-inch attenuation resistivity log and the other a relatively shallow buttonresistivityFigure 2.Well logs in (a)the well Atwater Valley 13#1and (b)the well Atwater Valley 14#1.Figures (a)and (b)both feature the difference between the bit size and the density caliper (blue shading,left track)and an image log derived from the GVR6tool (right track).(For interpretation of the references to color in this figure legend,the reader is referred to the web version of this article.)R.Birchwood,S.Noeth /Marine and Petroleum Geology 34(2012)186e 208189log,are shown.The shallow resistivity log shows spikes whenever gas hydrates are present in breakouts.However,at most of these spikes (shown by blue arrows)the deep-reading log does not show a signi ficant increase in resistivity.This suggests that the hydrates in the resistive breakouts are con fined to the near-wellbore region.Therefore the more plausible of the two possible explanations is that hydrates formed inside the breakouts during drilling.However,the time-span available for hydrate formation would have been very short.The time between opening of the hole and the pass of the GVR tool would have been anywhere between 1and 5min,depending on the rate of penetration (ROP).Why would hydrates form inside the breakouts?Possible explanations include:1.Enhanced surface roughness inside the breakouts is conducive to gas hydrate nucleation or accumulation.Surface roughness is thought to aid the nucleation of gas hydrates (Ceccotti,1966;Lee,2007).An additional effect of surface roughness may be to impede re-entrainment of deposited gas hydrate crystals intothe flow stream.The effect of surface roughness on detachment of particles from a surface by hydrodynamic forces is described in Vaidyanathan and Tien (1988)and Das et al.(1994).2.The presence of breakouts could alter the flow pattern to produce conditions more favorable to gas hydrate formation and/or deposition.When fluid flows over a cavity,the pressure inside a cavity can be reduced relative to that in the undis-turbed flow and velocity reversal can occur (Sinha et al.,1981;Zdanski et al.,2003).Such conditions may aid formation and deposition of gas hydrate crystals.The fact that the annulus between the BHA and the borehole wall was narrow 3should have enhanced the effect of breakouts on the flow regime.3.Gas hydrates tend to form everywhere but they are constantly removed from the borehole wall as a result ofmechanicalFigure 3.Image log derived from GVR6tool in Atwater Valley 13#1.Hole diameter (blue curve),gamma ray (green curve),and hydrate saturation derived from resistivity (shaded)shown in leftmost track along with measured depth.Image derived from shallow resistivity reading shown in Track 2.Hydraulic fractures appear as conductive features running parallel to the vertical borehole.(For interpretation of the references to color in this figure legend,the reader is referred to the web version of this article.)3The average annular gap between the GVR6tool and the in-gauge borehole wall was approximately 0.9in.R.Birchwood,S.Noeth /Marine and Petroleum Geology 34(2012)186e 208190agitation of the fluid by the rotating drill pipe.Gas hydrates inside breakouts are relatively protected from these effects due to their increased distance from the drill pipe.It is obvious that more rigorous studies of this phenomenon need to be carried out.4.Geological origins of stress anisotropyBorehole breakouts and drilling-induced fractures at both Atwater Valley and Walker Ridge are indicative of a difference in the magnitudes of the maximum and minimum horizontal stresses.The apparent maximum stress direction in the Atwater Valley sites is approximately east e west,while the direction in the Walker Ridge drilling site rotates from northwest e southeast at the surface to north-northeast e south-southwest deeper in the subsurface.It will be shown that the stress state in Atwater Valley is quite complex and possibly non-uniform.To gain insight into the controls on stress direction in both areas,bathymetric maps and seismic cross-sections were examined.Yassir and Zerwer (1997)observed that the maximum horizontal stress in the Gulf of Mexico is frequently aligned with bathymetric contours that describe the prograding sedimentary wedge of the Gulf of Mexico (Yassir and Zerwer,1997).However,they caution that topographic features by themselves are unlikely to exercise much control over stresses except very near to the surface.Simulations of stresses in a uniform half-space suggest that the in fluence of typical surface topographical features on horizontal stresses dies out within a few hundred meters of the surface (Evans et al.,1989).Caution is therefore warranted in interpreting stress orientations in the JIP wells based solely on surface topography.Stress orientations derived from surface features are more likely to be indicative ofstresses at depth to the extent that they represent the overall geometry of deeper structural features.The shape of the sedimentary wedge in the Gulf of Mexico allows for the extensional stress regime on the continental slope with typical listric normal faulting,gravity spreading and gravity gliding,and a compressional stress regime at the toe of the wedge on the lower slope with reverse or thrust faults (Galloway,1986).In such a geotectonic setting,the direction of maximum horizontal stress would be parallel to the strike of the normal faults (Anderson,1951).This means that on a regional scale the maximum horizontal stress is oriented in a general northeast-southwest direction.Examples are shown in studies by Yassir and Zerwer (1997)and recently by Cook et al.(2008)for Keathley Canyon and by Cook (2010)for the Walker Ridge sites.However Cook (2010)treated the stress direction in WR313-G to be uniform and did not consider the subtle expressions of stress rotation with depth in this well.It will be shown that such rotations are probably caused by the interplay between the structure of the salt minibasin penetrated by WR313-G and the compressional forces exerted by the surrounding salt.4.1.Atwater ValleyFigure 9shows a bathymetric map of the Atwater Valley area with the direction of maximum horizontal stress as can be inter-preted from the drilling-induced fractures in AT13#1and AT14#1.The interpreted stress direction is not aligned with bathymetric contours and is therefore at variance with the trend observed by Yassir and Zerwer (1997)in the Gulf of Mexico.One possible cause could be salt-induced stress rotation.Figure 10shows seismic cross-sections through the trajectory of the well AT13#1.A series of faults extend upwards to the sea floor from the salt body.In general the area is characterizedbyFigure 4.Image logs revealing drilling-induced fractures in WR313-G.R.Birchwood,S.Noeth /Marine and Petroleum Geology 34(2012)186e 208191northwest e southeast trending faults that provide pathways for gas to migrate to the surface.Many of these faults intersect the mudline 4and appear to be active.Salt migration in the underlying northwest e southeast-trending salt ridge caused faulting and warping of the overlying sediments,resulting in steeply dipping beds (see Fig.10).Both AT13#1(Fig.10)and AT14#1intersected a major northeast-dipping fault that is in contact with the underlying salt,indicating that possibly,the stresses at these locations are in flu-enced by the salt body in the subsurface (Fig.10).Salt introduces a complex set of stress perturbations into the surrounding sedi-ments including enhanced horizontal stress anisotropy (Dusseault et al.,2004;Fredrich et al.,2003).The northwest e southeast-striking normal faults with northeast dip indicate maximumhorizontal stresses in a northwest-southeast direction;however,the drilling-induced fractures observed in the wells indicate an east e west direction.The faults extend all the way to the seabed and should thus provide a good indication of the present-day stress state.To understand the discrepancy,a detailed numerical model of stress perturbations around salt and faults is required.Such a study lies beyond the scope of the present paper.4.2.Walker RidgeFigure 11shows an east-west cross section of the minibasin in which the Walker Ridge wells were drilled.The different maximum horizontal stress directions,i.e.,northwest e southeast shallow,north e south from 7030to 7300ft RKB,and north-northeast e south-southwest in the deeper sections are described in Section 3.3.Thrust faults outcropping at the surface and associated with recent salt movement (Hutchinson et al.,2009;McConnell and Kendall,2002)indicate a direction of maximum horizontal stress consistentwithFigure 5.Drilling-induced breakouts in the well WR313-G recorded by the GVR6tool.Tracks 2,3,and 4show resistivity images at shallow,medium,and deep depths of inves-tigation,respectively.Borehole breakouts appear as conductive (dark)features running parallel to the vertical borehole and are consistently oriented in an east e west direction,indicating a maximum stress direction of north e south.Connection points at 7113and 7178ft RKB are shown.Note persistence of breakouts at all three depths of investigation.4The term “mudline ”refers to the boundary between the earth and a body of water.In marine settings the mudline coincides with the sea bed.R.Birchwood,S.Noeth /Marine and Petroleum Geology 34(2012)186e 208192。