Diagenetic Gypsum Research Articles

Formation and Early Diagenesis of Varved Evaporite Sediments in a Coastal Hypersaline Pool

ABSTRACT This paper reports an occurrence of varved gypsum-organic sediments from a recent shallow-water hypersaline pool in Ras-Mohamad, Southern Sinai. Primary deposition and diagenetic features in this deposit were followed by studying the mineralogy, the depositional textures, the interstitial brine composition, and the concentrations of ions coprecipitated with gypsum. During the summer, when evaporation is intense, gypsum is deposited on the bottom of the pool. During the winter evaporation is milder and the brine became undersaturated with respect to gypsum. At these lower salinities a carpet of blue-green algae floors the pool. The sediments of the pool are composed of alternating layers of gypsum mush and algal mats. The composition of the interstitial brine is essentially that of evaporated sea water, but the concentrations of Ca and SO4 in the upper layers are anomalously high. The calculated degrees of supersaturation with respect to gypsum are as high as about 2. The supersaturation was relieved in the lower layers by early diagenetic growth of gypsum. Some of the diagenetic gypsum has grown by displacement within the algal layers This growth eventually destroyed the fabric of the algal layer, which was ultimately erased. The diagenetic growth of gypsum is reflected also in the concentrations of cations coprecipitated with gypsum. The coprecipitated-ion concentrations decrease with depth, and this is interpreted as due to the decrease in the average growth rate of the crystals.

AN EXPERIMENT IN SOIL GEOCHEMICAL PROSPECTING FOR PETROLEUM, DELLA GAS FIELD, COOPER BASIN

The aim of the experiment was to test the method of prospecting for petroleum by soil geochemical analysis of hydrocarbons and, in particular, its possible application in the Cooper Basin. Some 379 samples were taken in traverses across the Della Gas Field and a nearby, undrilled anticlinal structure. The samples were taken from a depth of 5 m at a spacing of 5 per mile (approximately 1 every 300 m). Hydrocarbons were extracted with 2N HCI and analysed by gas chromatography. Some modifications to the published analytical techniques had to be made to overcome the prbblem of traces of organic compounds present in local waters and reagents.Arithmetic mean values analysed for the Della Gas Field were: acid soluble material, 8.62 per cent; ethane, 1.33 ppm; propane, 0.77 ppm; butane, 0.62 ppm; pentane, 0.84 ppm. In plots of, firstly, the sum of ethane, propane, butane and pentane and secondly that sum normalized to the amount of acid soluble material, anomalous areas containing only a few sample points occur above and near the Della Field, and the undrilled structure.There is only low statistical correlation between the amount of acid soluble material and the hydrocarbon content of the soils. The hydrocarbon content of the soils may possibly be associated with diagenetic gypsum and calcite and possibly smectite (?mixed layer clay) but the evidence is inconclusive. The soil hydrocarbon content shows some tendency to correlate with the finer grain sizes. The anomalies appear to bear no relation to the vegetation. A radiometric survey with a gamma ray spectrometer showed little variation of values and no relation to the geochemistry nor the outline of the Della Gas Field.The experiment has shown that significant variations in hydrocarbon content of the soils can be analysed in the Cooper Basin and apparent anomalous zones can be outlined. The anomalous zones may be related to the Della Gas Field.