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doi:10.2204/iodp.proc.308.105.2006 Downhole measurementsHole U1321A was a LWD/MWD hole and the only hole drilled at Site U1321. The objectives were to:
Logging while drilling and measurement while drillingOperationsThe same bottom-hole assembly and tool configuration assembled for Sites U1329 and U1320 were used for LWD operations in Hole U1321A (see “Operations” in the “Site U1320” chapter). The hole was started using a rapid jet-in penetration and pump rates of ~55 gallons per minute (gpm) from 0 to 20 mbsf before retrieving the vibration-isolated television camera. A controlled ROP of 30 m/h, bit rotation of 50 rpm, and pump rates of ~60 gpm were used for drilling the upper 25 mbsf until all the LWD sensors were below the seafloor. Pump rates were then slowly increased, reaching ~400 gpm by ~42 mbsf, where the mud pulsing system began transmitting data to the surface. The hole was drilled to 140 mbsf. Logging data qualityMWD/LWD operations in Hole U1321A reached 140 mbsf to provide data coverage by all LWD tools through the turbiditic and hemipelagic stratigraphic units drilled at Sites U1319 and U1320. Figure F2 shows the quality control logs for Hole U1321A. The target ROP of 30 m/h (±5 m/h) was achieved except for the upper 6.5 m of the hole, where rapid jet-in exceeded 370 m/h. The ROP slowed to ~60 m/h by 20 mbsf, followed by a steadier rate. Overall hole quality at shallow depths was variable with several high caliper measurements. The density-derived caliper log is near 25 cm toward the bottom of the hole. The bulk density correction varies from –0.04 to 0.15 g/cm3 (mean = 0.01 g/cm3) (Fig. F2), which indicates that good-quality bulk density measurements were obtained except for intervals characterized by washouts. Log depths, in mbsf, for the LWD logs were constrained by identifying the gamma ray signal at the seafloor. For Hole U1321A, the gamma ray log pick for the seafloor was at a depth of 1462.9 mbrf, 0.9 m deeper than the drillers depth. The rig floor logging datum was 10.5 m above sea level for this hole. Annular pressure while drilling and equivalent circulating densityAnnular pressure while drilling (APWD) within the borehole was monitored during MWD operations as annular pressure in excess of hydrostatic (APWD*) and equivalent circulating density referenced to the seafloor (ECDrsf) (see discussion in “Array resistivity compensated tool” in “Downhole measurements” in the “Methods” chapter). ECDrsf decreased from 0 to 35 mbsf. At 35 mbsf, ECDrsf increased from 9.5 pounds per gallon (ppg) to 12.5 ppg, which is also reflected as an increase in the APWD* to 0.25 MPa (Fig. F3). We interpret that this increase is due to sediment from a 20 m thick sand interval loading the annulus. At ~55 mbsf the lithology changed to a clay-rich unit, and ECDrsf and APWD* returned to stable values as the cuttings were flushed from the borehole. The ECDrsf and APWD* profiles below 55 mbsf do not show significant anomalies, suggesting that we did not drill through any permeable, overpressured units and that the borehole remained intact during operations. ResultsThe GeoVision Resistivity (GVR) tool gamma ray log (GR) is highly variable in the upper 67 mbsf. Below 67 mbsf, GR values are nearly constant with a mean value of 72 gAPI, suggesting a clay-rich lithology. From the GR, we interpret a series of intercalated sand and clay intervals above ~67 mbsf. The most prominent stratigraphic marker is a 25 m thick sand-rich interval located between 35 and 60 mbsf, where gamma ray values are as low as 11.4 gAPI. GVR deep button resistivity increases from 0.3 to 1.9 Ωm from the seafloor to 140 mbsf. Resistivity variations are interpreted as changes in sand and clay content. The most prominent feature below 67 mbsf is a clay-rich unit between 85 and 93 mbsf that is bounded by 1 m thick silty sand intervals (Fig. F4). The Vision Density Neutron tool bulk density log increases from 1.0 g/cm3 at shallow depths to 2.0 g/cm3 near the bottom of the hole (Fig. F4). Neutron porosity decreases from 85% to 45% with the largest porosity fluctuations occurring in sand-rich intervals (Fig. F4). Density and neutron porosity data suggest normal compaction. Density and neutron porosity values above 67 mbsf reflect the presence of sand units and correspond to enlarged borehole dimensions. The photoelectric factor (PEF) log follows similar trends as those observed in the gamma ray, resistivity, and density data. The PEF log shows systematic variations in the upper 67 m that we interpret as sharp changes in clay and sand content (Fig. F4). Definition and interpretation of logging unitsWe conducted a preliminary lithologic interpretation in Hole U1321A based on gamma radiation, resistivity, and GVR resistivity-at-the-bit images (Fig. F5). Seismic correlation indicates that the lithology at Site U1321 should be similar to Sites U1319 and U1320. The correlation between lithology and log response at those sites shows that sands, clay, and silt/silty sands can be distinguished based on gamma ray and resistivity. GVR resistivity images at Sites U1319 and U1320 also show that relatively thin (~5 cm) sand and silty sand beds can be resolved in many cases. The nature of the bedding contacts, whether conformable or erosional, and, in some cases, contorted bedding can also be inferred from the GVR resistivity images at Site U1320. Given the observations and initial correlation between lithology and log response at Sites U1319 and U1320, we defined thresholds for clay, silt, and sand as follows:
At Sites U1319 and U1320 we penetrated hemipelagic foraminifer-bearing clays associated with lithostratigraphic Units III and V (at Site U1319). These clays had a slightly higher resistivity (up to ~1.5 Ωm) than adjacent clay intervals but similar gamma ray response. We used this distinctive signature to infer the presence of these hemipelagic intervals at Site U1321. Where thin beds are inferred from the GVR images, gamma radiation shows only minor deflections. The minor gamma ray response is due to bad resolution. Where thin beds are inferred, we infer grain size (silt or sand) from the RING resistivity curve, which has higher resolution than the gamma ray tool. Logging units at Site U1321 were defined primarily from the gamma ray and resistivity curves using 1468 mbrf as the depth to the seafloor (Fig. F5). We defined two major logging units, with the upper unit (logging Unit 1) divided into four subunits (1a, 1b, 1c, and 1d). Logging Unit 2 was not divided into subunits (Table T2). Logging Unit 1 (0–70.5 mbsf)Logging Unit 1 is defined by a highly variable gamma ray response ranging from ~15 to 75 gAPI and resistivity variation of 0.1 Ωm (Fig. F5). Logging Unit 1 is interpreted to contain five sand packages (P1–P5) interbedded with muddy intervals, with a net sand content of ~60%. In the uppermost 4 m of the hole (0–4 mbsf) we observe a gradual upward decrease in gamma radiation and resistivity. This is likely a consequence of hole enlargement and very porous sediments. With analogy to Hole U1320A, we interpret this interval to be silty with thin beds and laminae of sand, grading to a foraminifer-bearing clay at the top of the hole (Figs. F4, F5). Logging Subunit 1a (0–20 mbsf)Logging Subunit 1a contains sand Packages P1 and P2 with thicknesses of 6 m and 2 m, respectively (Figs. F4, F5). These packages have sharp bases and gradational tops. Package P1 probably has three or more amalgamated sand beds. The muddy interval between Packages P1 and P2 is interpreted to contain thin sand and silt beds. This subunit is defined by covariation of gamma radiation and resistivity curves over intervals of several meters. Porosity and density display similar variations that support interpretation based on the gamma ray and resistivity curves (Fig. F4). Logging Subunit 1b (20–32.5 mbsf)Logging Subunit 1b is defined by a uniformly high gamma ray and resistivity response interpreted as predominantly mud. A slightly lower gamma ray response at the base of the subunit suggests higher silt content (Figs. F4, F5). Density has a pattern similar to those exhibited by the gamma ray and resistivity data, whereas porosity is predominantly lower than within the subunits above and below. Logging Subunit 1c (32.5–60.5 mbsf)Logging Subunit 1c is 26 m thick and consists almost entirely of sand Package P3 (Fig. F5). This package has a sharp base and a gradational top. At the top of the subunit, the GVR resistivity image suggests dipping surfaces, which are either related to erosional scours or deformed/folded beds. Within the package, subtle resistivity changes are interpreted to represent amalgamation surfaces. In the upper part of sand Package P3 we observe high resistivity in the GVR resistivity images (Fig. F5) that are interpreted as mud clasts. This subunit is defined by resistivity and gamma ray responses that drop over the upper 10 m of the interval and then remain low to the base. Density decreases and remains low throughout this subunit, whereas the opposite response is observed in porosity (Fig. F4). Logging Subunit 1d (60.5–70.5 mbsf)Logging Subunit 1d is composed of sand Packages P4 and P5, each ~2 m thick (Figs. F4, F5). The packages have sharp bases, gradational tops, and appear to be composed of one to two sand beds each. Mud intervals within logging Subunit 1d appear to show laminations and contorted beds (Fig. F6). In this subunit, resistivity and gamma ray values vary over intervals of a maximum of 2–3 m. Density and porosity also show similar responses throughout this subunit (Fig. F4). The top of this subunit is characterized by a thin clay-rich interval. Based on the check shot data from Hole U1320A, it appears that this clay-rich interval correlates with seismic Reflector R20. Logging Unit 2 (70.5–140 mbsf)Logging Unit 2 is dominated by clay and mud. Based on slightly higher resistivity, we interpret two intervals of hemipelagic clays at or below seismic Reflectors R30 and R40. The upper hemipelagic clay interval is ~8 m thick, and the lower interval is ~4 m thick. These intervals correlate well with foraminifer-bearing clays cored below seismic Reflectors R30 and R40 at Site U1319 and below seismic Reflector R30 at Site U1320. The clay interval between 78.5 and 87 mbsf is interpreted to have thin sand or silt beds based on the resistivity data. Below 87 mbsf, logging Unit 2 appears to have small-scale variations in resistivity that are interpreted to represent centimeter- to decimeter-scale variations in clay and silt content. Steeply dipping surfaces found within this unit may represent deformation due to mass transport deposits or faulting (Fig. F6). Overall, logging Unit 2 shows little variation in gamma ray and resistivity response. Gamma ray values generally track at ~75 gAPI, with a few excursions reaching to slightly less than 60 gAPI. Resistivity is relatively high and uniform, generally tracking just higher than 1 Ωm. From 70.5 to 78.5 mbsf resistivity is locally elevated above background and gamma radiation is also slightly higher than background over this interval. Similar local positive anomalies in resistivity and gamma ray values occur from 87 to 94 mbsf. This latter feature is bound from 85 to 87 and 94 to 96 mbsf by the two significant excursions in gamma ray values. Physical properties from log dataAt Site U1321, LWD data provide estimates for sediment physical properties. Downhole profiles of density and density-derived porosity were used for stress analyses at this site assuming hydrostatic conditions. Density and porosityThe LWD bulk density profile from Hole U1321A is used to derive porosity (Fig. F7A):
where
The bulk density profile shows an overall increase with depth from 1.0 to 2.0 g/cm3, which equates to porosity estimates decreasing from 85% to 45% (Fig. F7A). Based on these profiles, four distinct intervals can be distinguished in Hole U1321A. These intervals are highlighted by washouts and interpreted as sandy layers. Therefore, the data in the intervals underestimate the true bulk density. Relatively low bulk densities averaging ~1.35 g/cm3 and high porosities (~85%) characterize the upper part of logging Subunit 1a (0–11 mbsf). A second interval, interpreted to be a clay-rich section, comprises the lower part of logging Subunit 1a, all of Subunit 1b, and the upper part of Subunit 1c (11–37 mbsf). This interval has higher bulk densities (~1.7 g/cm3) and lower porosities (~60%). The third interval, including most of logging Subunit 1c (37–59 mbsf) has lower densities (~1.25 g/cm3) and high porosities (85%–90%). At 60 mbsf, a sharp increase in bulk density, which correlates with seismic Reflector R20, marks the beginning of the fourth interval. In this interval there is a gradual increase in bulk density from ~1.8 to 1.9 g/cm3. A few minor density peaks can be observed in this interval, and seismic Reflector R40 correlates with one of these peaks at 85 mbsf. A vertical hydrostatic effective stress (σvh′) profile can be obtained from the bulk density (see “Physical properties” in the “Methods” chapter). The high variations observed in the bulk density data (Fig. F7A) are translated to a nonlinear vertical effective stress profile (Fig. F7C). If bulk density is filtered to remove anomalously low bulk density (ρb < 1.5 g/cm3 from 0 to 20 mbsf and ρb < 1.75 g/cm3 below 20 mbsf), the vertical hydrostatic effective stress profile is near linear (Fig. F7C). These thresholds account for enlarged borehole dimensions and are based on density measurements from Sites U1319 and U1320. Bulk density and porosity measured at the three Brazos-Trinity Basin IV sites suggest that the sediments from Hole U1321A below seismic Reflector R40 are less consolidated than those at Site U1320 but are more consolidated than those at Site U1319 (Fig. F8A). These differences in consolidation are most likely due to the difference in overburden pressure at the three sites. However, at the same vertical hydrostatic effective stress, sediments below seismic Reflector R40 at Site U1319 have lower porosity than those at Sites U1320 and U1321 (Fig. F8B). If the sediments from all sites have the same compaction properties, then under hydrostatic conditions they should all plot along the same vertical hydrostatic effective stress-porosity curve. These results could indicate underconsolidation at the two sites located within the basin (Sites U1321 and U1320). Deviations of these trends could be controlled by overpressure or lithologic variability. Core-log-seismic integrationThe largest variations in LWD log responses occur from 0 to 67 mbsf, an interval that corresponds to logging Subunits 1a, 1b, 1c, and 1d. These subunits are interpreted to be interbedded sand and clay layers of varying thicknesses. These variations allow extrapolation between core and seismic observations. Four regional reflections (seismic Reflectors R10, R20, R30, and R40) mapped on high-resolution 2-D seismic data have been correlated with log data from Hole U1321A. Seismic Reflector R10 correlates with small decreases in density, resistivity, and gamma radiation within a predominantly clay rich interval, suggesting that a ~2.0 m thick sand interval is creating the reflection (Fig. F9). Seismic Reflector R20 correlates to the base of the first sand unit (~1.5 m thick) within logging Subunit 1d at ~61.6 mbsf. This thin interval correlates with a sand layer within lithostratigraphic Unit II at Site U1320, located at ~110 mbsf. Seismic Reflector R30 was picked on a trough on the seismic lines; however, the base of the turbidite succession in Hole U1321A occurs at the peak immediately above (Fig. F9). The most prominent feature within logging Unit 2 is a clay-rich layer located between 85 and 93 mbsf that is bounded by 1 m thick silty-sand intervals. The top of this interval correlates with seismic Reflector R40. LWD data are linked to seismic data through a time-depth conversion. We used LWD density data (not corrected for washouts) and a constant velocity (1600 m/s) to construct a synthetic seismogram for Hole U1320B. A 200 Hz minimum-phase Ricker wavelet was convolved with the reflection coefficients to create the synthetic seismogram. The correlation of events between the synthetic seismogram and the high-resolution 2-D seismic data indicates that the time-depth model is appropriate for these sediments. An accurate time-depth model allows correlation of seismic reflections with observations in core and logging data. SummarySite U1321 logging data provide a detailed picture of the lithofacies present at the southern edge of Brazos-Trinity Basin IV. Detailed core-log integration at Sites U1319 and U1320 will enable refinement of our preliminary interpretation, which is as follows:
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