M. Julleh Jalalur Rahman

The composition of the subsurface Neogene shales of the Surma group from the Sylhet Trough, Bengal Basin, Bangladesh

Abstract Nineteen shale samples from six wells penetrating the Neogene Surma Group in the Sylhet Trough in northeastern Bangladesh were analyzed by X-ray diffraction, X-ray fluorescence spectrometry and direct current plasma optical emission spectrometry. Mineralogically, the subsurface Neogene shales of the Surma Group comprise predominantly quartz, plagioclase, illite, chlorite, kaolinite, with lower amounts of K-feldspar, dolomite and smectite, and minor to trace amounts of calcite and siderite. Significant variation in the content of K-feldspar, smectite and illite–smectite mixed layer clays with depth has been observed. K-feldspar has not been noted below the depth of about 3200 m, and smectite has not been observed deeper than 3000 m. It is inferred that illite–chlorite, quartz and feldspar are predominantly detrital in origin, reflecting a fast-rising source terrain with no intense weathering activity. Geochemically, the mean major element composition of the shales is similar to that of the average shale with the exception of the CaO content, which is lower here. The low CaO content could be due to lower carbonate and total organic content. The samples are enriched in Sc and Cu and are depleted in Be, Sr and La. Other trace elements have values close to the average shale. The geochemical characteristics suggest that the Neogene shales of the Surma Group were derived from an active margin setting related to a recycled source.

40Ar/39Ar multigrain dating of detrital white mica of sandstones of the Surma Group in the Sylhet Trough, Bengal Basin, Bangladesh

Abstract The Sylhet Trough, a subbasin of the Bengal Basin in northeastern Bangladesh, is bordered by two orogens, the Himalaya to the north and the Indo–Burman Ranges to the east. Detrital white micas from six sandstone samples of the Neogene subsurface Surma Group were analysed by the 40Ar/39Ar method. The ages of the individual steps in the Ar/Ar release diagrams range from 25 up to 92 Ma. These ages represent cooling ages rather than formation ages because they originate from large mica flakes. The low-temperature step Ar/Ar ages of detrital white mica, with plateau-type character, are diagnostic for the youngest cooling events and cluster between 25 and 36 Ma, except one age of 51 Ma. The medium- and high-temperature steps of the Ar-release diagrams are characterized by staircase-type pattern. These various age discordances are interpreted as a mixture of micas with different ages. The 40Ar/39Ar results point to heterogeneous ages of the source region, which seems to be typical of the early stages of a rapidly uplifting collision orogen. The narrow age range of the low-temperature steps can be correlated with early cooling ages known from the High Himalayan Crystalline basement. The Ar/Ar data indicate that the major sediment input in the Sylhet Trough during sedimentation of the Surma Group has been derived from the rising Himalaya orogen.

Geochemistry and provenance of the Miocene sandstones of the Surma group from the Sitapahar anticline, Southeastern Bengal Basin, Bangladesh

Geochemical composition (major and trace elements) of Miocene sandstones of the Surma Group exposed in Sitapahar anticline, Southeastern Bengal Basin was determined to reveal their provenance, tectonic setting and source area weathering conditions. The sandstones are sub-arkosic, sub-lithic and greywacke in composition with abundant low-grade metamorphic, sedimentary lithics (mainly chert with some shale fragments), low feldspars and little volcanic detritus. Compared to the average sandstone value, the Surma Group sandstones are depleted in CaO and enriched in Al2O3, Fe2O3 and Na2O. The Chemical Index of Alteration (CIA) values for the Miocene Surma Group sandstones vary from 57 to 73 with an average of 65, indicating low to moderate weathering of the source areas. The geochemical characteristics suggest an active continental margin to passive margin setting for the Surma Group sandstones; preserve the signatures of a recycled provenance that is agreement with sandstone petrography and derivation of these sandstones from felsic source rocks.

Facies characterization of the Surma Group (Miocene) sediments from Jalalabad gas field, Sylhet trough, Bangladesh: Study from cores and wireline log

The Sylhet trough located on the north-eastern margin of present Bengal basin, contains ~22 km of Tertiary sediments and well known as a hydrocarbon producing province. A detailed facies characterization of the subsurface Miocene Surma Group sediments (especial emphasis on reservoirs sandstones) from Jalalabad gas field within the Sylhet trough has been done using core log analysis and wireline log (gamma ray) interpretation. Texture and sedimentary structures of the cores suggests that the nine individual lithofacies types which can be grouped together into three facies associations, namely, fine-grained facies associations (FFA), medium-grained facies association (MFA) and coarse-grained facies associations (CFA). Major changes in gamma ray log motifs and various bounding discontinuities indicate six para-sequence sets (basin wide) and twenty eight para-sequences (local environmental changes) within the depth range from 2200-2800 m. Detailed facies analysis of the cores and wireline log reveals that the interbedding facies within the associations in the Surma Group commonly develop small-scale fining-upward (FU) cycles, coarsening-upward (CU) and random intercalations (RD). The sediments of the Surma Group of the Jalalabad field have been interpreted as deposits of the shallow marine to tide-dominated deltaic depositional setting. The cyclic nature of sedimentation pattern of the Surma Group probably records an almost continuous existence of this prograding deltaic regime and a tectonic setting characterized by a mixture of prolonged basin subsidence and regional transgression coupled with sporadic regressive phases.

Mineralogy and chemical composition of the clay fraction of Neogene shales from the Surma Group in the Bengal basin, Bangladesh

Mineralogical and chemical investigations (<2μm clay separates) of shale samples from the Neogene-age Surma Group obtained from four wells (Habiganj-11, Shahbazpur-1, Titas-11, Titas-15) in the Bengal basin, Bangladesh, were carried out in order to reveal the clay mineral composition as reservoir exploration and exploitation requires a good understanding of the clay minerals. The samples were analyzed by X-ray diffraction (XRD), scanning electron microscope (SEM) and X-Ray fluorescence spectrometry (XRF). Mineralogically, the sub-surface Surma Group shales comprise predominantly quartz, plagioclase, illite, chlorite, kaolinite, with lesser amounts of K-feldspar, dolomite and smectite, and minor to trace amounts of calcite, siderite and pyrite. The chemical composition of the <2 μm clay separates also suggests an illite and chlorite-rich composition. With increasing burial depth, the Surma Group shales are enriched in illite. The gradual decreasing of the smectite clays with depth and ultimate disappearance at greater depths (≥ 3000 m) may have been responsible for the presence of the diagenetic illite. Based on the mineralogical composition it is most likely that the illite-chlorite associations together with quartz and feldspar were predominantly detrital in origin and thus reflect the presence of a rapidly-rising source terrain not subjected to intense weathering.