Coastal & Marine Geology Program >Subsidence and Fault Activation . . . > Open File Reoprt 01-274

Shallow Stratigraphic Evidence of Subsidence and Faulting Induced by Hydrocarbon Production in Coastal Southeast Texas

USGS Open File Report 01-274

Robert A. Morton, Noreen A. Purcell, and Russell L. Peterson

Contents: Summary Introduction Geology & Production Histories Sediment Surface Profiles Sediment Cores Results Coastal Environmental Implications References Appendix A Appendix B Project Contact: Bob Morton

Sediment-Surface Profiles Topographic and bathymetric profiles (Figs. 2, 3, 4, Appendix A) were surveyed independently with portable field equipment including a tripod-mounted level, graduated rod, and GPS receiver. Relative heights for ground elevations were obtained by establishing transects across the marsh at selected sites. Static GPS positions were used to determine the geographic coordinates of the start and end of each transect. In open-water areas, water depths were measured from a small boat with a graduated rod, whereas the geographic coordinates of each depth measurement were obtained with the GPS receiver. The independent topographic and bathymetric surveys were linked to construct continuous sediment-surface profiles (Figs. 5, 6, 7) by using simultaneous water-level measurements in the marsh and open water as a common datum. Fig. 2 Fig. 3 Fig. 4 Fig. 2. Locations of shallow cores (numbers) and sediment-surface profiles (letters) surveyed at the Port Neches Field. Also shown in color are the three areas used to calculate accommodation space created by subsidence and erosion. [larger version] Fig. 3. Location of sediment-surface profile (purple square) surveyed at the Clam Lake Field. Historical wetland loss generalized from White et al. (1987). [larger version] Fig. 4. Locations of shallow cores (numbers) and sediment-surface profiles (letters) surveyed at the Caplen Field. Also shown in color are the three areas used to calculate accommodation space created by subsidence and erosion. [larger version] Profile Adjustments Water depths measured in the field can be compared only if they are corrected for tidal stage or any unusual conditions (such as flooding) that would bias the data. The Conrad Blucher Institute (CBI), Division of Nearshore Research, at Texas A&M University - Corpus Christi (TAMUCC) operates the Texas Coastal Observation Network (TCOON), which is a network of tide gauges located throughout the coastal waters of Texas. Using electronic tide gauge data from the CBI website, primary water levels for the dates and times of data collection were plotted relative to mean higher high water (MHHW) at the nearest tide gauge (Fig. 1) in the TCOON database. Water levels above or below MHHW during the times of data collection were used to adjust the field measured water depths to a common datum. MHHW was selected as the common datum for comparison because it represents the flooding surface to which wetlands aggrade when they are in equilibrium with the extant coastal processes. Furthermore, the digital water-level records provided by CBI are elevations relative to MHHW, so no additional corrections were necessary for the tidal datum analyses. Water-depth measurements at Port Neches Field were adjusted to the MHHW datum from the Rainbow Bridge Station, whereas water-depth measurements at Caplen Field were adjusted to the MHHW datum from the Rollover Pass Station (Fig. 8). Tide gauge records at the Rainbow Bridge Station, show that average water levels for 06/18/00 were +0.06 m above MHHW (Fig. 8). Consequently, the water depths measured at Port Neches were adjusted downward by 0.06 m. The tide gauge records at Rollover Pass for 06/20/00 and 06/21/00 indicate that average water levels were +0.12 m and +0.01 m above MHHW, respectively, during the field operations (Fig. 8). Therefore, the appropriate values were subtracted from all water depths measured on the respective days. The TCOON water-level data confirm field observations at Caplen that tidal conditions were higher on 06/20/00 than on 06/21/00. Profile Descriptions Bathymetric profiles across the Port Neches Field show that water depths of slightly less than 1 m are relatively uniform except at the marsh edges or where dredged material has been deposited along a channel (Fig. 5). Average water depths are only slightly greater along profile O-P (Appendix A), which runs roughly parallel to a reactivated fault that is downthrown to the northwest (White and Morton, 1997). Together the Port Neches sediment-surface profiles describe a bowl-shaped area of wetland loss with relatively steep sides at the margins and relatively uniform depths within the bowl. Fig. 5 Fig. 6 Fig. 7 Fig. 5. Sediment-surface profiles AB an OP from the Port Neches Field. Profile locations shown on Fig. 2. [larger version] Fig. 6. Sediment-surface profiles from the Clam Lake Field. Profile locations shown on Fig. 3. [larger version] Fig. 7 Sediment-surface profiles AB and IJ from the Caplen Field. Profile locations shown on Fig. 4. [larger version] At Clam Lake, the topographic profile was positioned so that it crossed a long reactivated fault segment (Fig. 3). Along the transect, plant density is high, and local variations in elevation can be caused by clumps of roots as well as changes in ground level. Because the local ground-level variability is high, the relative heights of the profile do not show the exact location of the fault (Fig. 6). Instead, the general slope of the profile, average heights of beginning and ending segments, and change in predominant wetland vegetation define the region of the fault. The lateral change in marsh vegetation from Scirpus spp. (bulrush) to Typha spp. (cattail) coincides with slightly lower heights and wetter conditions on the downthrown side of the fault. The marsh and open-water profiles at the Caplen Field (Fig.7) show that the sediment-surfaces perpendicular to the fault traces are asymmetrical. Slopes from the marsh to open water are steep across the faults, but slopes are gradual away from the faults (see Appendix A, profiles AB, IJ, and MN). Maximum water depths at Caplen away from the marsh edge or dredged material are typically about 0.6 m. Water bottoms near the faults consist of firm mud, and oyster shells are commonly present. Estimating Open-Water Accommodation Space Individual topographic and bathymetric transects were selected to represent the spatial distribution of marsh and open water in each area of subsidence and fault reactivation. Measurements at all data points visited in the field were included in the topographic and bathymetric profiles; however, an 'x' on the profile (Appendix A) identifies a data point that was not included in the calculation of average water depths along the bathymetric profile. Calculations of average water depth and open-water accommodation space do not include these data points because they are either adjacent to or on the natural marsh surface, or on the surface of dredged material. Topographic gradients are relatively steep between the marsh and adjacent open water, and inclusion of water depths at the marsh edge would bias (decrease) the average depths of the open water and underestimate the volume of open-water accommodation space. Fig. 8. Water levels recorded at the (A) Rainbow Bridge and (B) Rollover Pass tide gauges during the field investigations. Locations of tide gauges shown on Fig. 1. [larger version] To estimate the volume of open-water accommodation space, each field site (Caplen and Port Neches) was divided into three sectors. The area (m2) for each sector was determined by drawing polygons around the sectors on a digital quadrangle map (Caplen) and on a digital orthophoto quarter quadrangle (Port Neches). The open-water accommodation space for each sector was calculated by multiplying the surface open-water area (m2) and the average water depth (m) for that area. Considering the potential error in water-level adjustments from tide gauge records (a few cm) and the potential error in each water-depth measurement (0.5 cm), it is estimated that the maximum potential error in accommodation space is about 150,000 m3 for the largest open-water sectors (1 and 3) of the Port Neches Field. Errors in estimating accommodation space in all three sectors at Caplen and in sector 1 at Port Neches are probably substantially less than the maximum potential error because the areas of these latter sectors are less than half of the largest open-water areas at Port Neches. Accommodation space associated with wetland loss at the Clam Lake Field was not systematically estimated because no sediment-surface profiles were obtained in the open-water areas. Nevertheless accommodation space there can be approximated by taking the area of open water (275 hectares) measured by White and Tremblay (1995) and inferring that average water depths are about 0.4 m based on the average water depths measured at Caplen. Those values yield an estimated accommodation space of 1,100,000 m3. The three sectors of wetland loss at Caplen (Fig. 4) were divided by drawing polygons: 1) along the east-west trending fault trace, 2) along the north-south trending fault trace, and 3) at the eastern side of the dredged canal (Sun Oil Company). The open-water accommodation spaces for sectors 1, 2, and 3 at Caplen are 943,400 m3, 729,100 m3, and 1,794,200 m3, respectively. The total volume of accommodation space in the Caplen Field is about 3,466,700 m3. Transect locations and road segments that are still exposed controlled the division of Port Neches into three areas of wetland loss. The open-water accommodation spaces estimated for sectors 1, 2, and 3 at Port Neches are 6,623,400 m3, 2,492,600 m3, and 6,210,800 m3, respectively. The total volume of accommodation space in the Port Neches Field is approximately 15,326,800 m3. « Geology & Production Histories | Sediment Cores »

Coastal & Marine Geology Program >Subsidence and Fault Activation . . . > Open File Report 01-274