Investigations of surficial sediments, suspended particulates and volatile hydrocarbons at Buccaneer gas and oil field

Abstract

From the Abstract: “This report describes the results of a four-year study of the surficial sediments, suspended particulate material and volatile organic hydrocarbons around structures in the Buccaneer Gas and Oil Field in the Gulf of Mexico. although the present investigators were responsible for only the last two years of the project, all four years of the work are summarized. More than five thousand samples were collected and analyzed during the third and fourth years of the study alone.

During the course of this study, large seasonal variations in the suspended material in the water column were observed. These seasonal differences were evident in both the concentration of total suspended matter (TSM) and in the form of the suspended material present in the water column (e.g. clay vs. plankton). The seasonal variations are directly related to several factors including: stratification of the water column, primary productivity, effect of inputs from bays and estuaries, and bottom nepheloid layers concurrent with bottom currents in the study area. These are all natural processes, and the variations in parameters observed are typical of the seasonal changes observed throughout most of the nearshore Gulf coastal area.

The Buccaneer Gas and Oil Field production platforms do not measurably alter the bulk composition of suspended particulates or biological activity (as measured by ATP and chlorophyll) in the water column in their immediate vicinity. These results can be easily explained by the fact that small brine volumes are discharged by the platform compared to the large volumes of sea water transported through the field by natural processes. In the surficial sediments, decreasing gradients in organic and inorganic carbon away from the platforms were observed. This is the type of distribution that would be expected if a higher degree of productivity is associated with the platforms, a real possibility. Bottom sediments in the Buccaneer Field area consist generally of relict calcereous shell hash, silts and clays of the Beaumont Formation, and sands which are dominated by a strong 3.5~ fraction.

Most sediments ranged from extremely poorly sorted to poorly sorted, with an increase in sorting with distance away f rom the platf orms. A general decrease in sediment grain size was also found with increasing distance from the platforms. This sedimentological data indicate that there is considerable movement of fine grain material in the area. The presence of near bottom nepheloid layers, as observed by transmissometry during all seasons, confirms the conclusions of the sediment grain size analyses. The fine-grained,surficial sediments within the Buccaneer Field are probably in a continued state of resuspension and reworking. If contaminates from the production platforms were introduced into the sediments, only those contaminates associated with very coarse grained material would be expected to remain permanently in the field.

Gaseous (CI-CO hydrocarbons measured in the water column in the Buccaneer Field were typical of unpolluted waters along the Texas coast. Any input of gaseous hydrocarbons in the water column seems to be rapidly diluted. The gaseous hydrocarbons in the sediment similarly showed no discernable influence due to production platforms. The methane levels seemed to be controlled mainly by sediment particle size, with highest levels associated with a high sediment clay content.

There were significant seasonal and daily variations in the concentration of gaseous and volatile liquid hydrocarbons (:VLH) in the Buccaneer oil and brine. In-general, one-ring aromatic compounds (e.g., benzene, toluene, ethylbenzene, xylenes) are the major hydrocarbon components in the brine discharge. Benzene, the most soluble VLH, comprised from 40 to 50 % of the volatile hydrocarbons. Taken together, the one-ring aromatics comprised 70 to 90% of the VLH in the brine. VLH n-alkanes comprised only 2 to 10% of the VLH in the brine. Although the concentrations of total VLH in the brine varied considerably, one-ring aromatics were always the major fraction. The average aromatic VIE concentration in the brine using our four samples was 16,600 jig/L. Assuming a mean rate of discharge of brine at 95,400 liters per day, the mean rate of discharge of VLH aromatic compounds is 1580 g per day. This compares to 191 g of high-molecular-weight n-alkanes discharged (Middleditch et al., 1978).

At a grid of stations between 25 and 50 meters from the brine discharge, VLH ranged from 128 to 6400 ng/L, depending on the current direction. The component composition of the VLH fraction also varied between these stations, indicating different sources of origin for various components. At Platform 288A (no brine discharge), VLH were considerably lower, ranging from 169 to 374 nl/L and consisted of either aromatics or n-alkanes. Low-molecular-weight hydrocarbons near both platforms showed a similar distribution as VLH, indicating a common source--the discharge brine.”