Applying high resolution sequence stratigraphy in lacustrine deltaic reservoirs of the Pojuca Formation, Recôncavo Basin (Northeast Brazil)

Abstract

This study presents a new stratigraphic and geochronological framework interpretation, based on the high resolution sequence stratigraphy technique, of the Miranga and Santiago operational zones in an oilfield in the Recôncavo Basin, in Bahia State. The interval studied includes the Miranga and Santiago production zones, belonging to the Pojuca Formation (Ilhas Group), deposited by lacustrine deltas during the Hauterivian in the rift phase of the Recôncavo Basin. This unit is constituted by predominantly fine sandstones, intercalated with siltstones, shales and carbonates. Some of the pelitic and carbonatic layers defines graphoelectrical markings traceable in the scale of the basin (Silva, 1993). Detailed analyses were carried out on the cores and well logs, leading the identification of facies and facies associations, the construction of sectionSV)s and maps, and interpretation of vertical successions, of stratigraphic surfaces and depositional trends. Twelve high-frequency vertical successions (SV) were identified, which represent pulses of deltaic sedimentation prograding over prodelta and deeplake facies. Then, the following stratigraphic surfaces were interpreted and named: maximum flooding surface (SIM), transgressive surface with possible wave ravinement (ST), maximum regressive surface (SRM) and sequence boundary (LS). This identification allows the naming of the system tracts: transgressive lake system tract (TSLT), high-stand lake system tract (TSLA), and low lake system tract (TSLB). These system tracts define 11 high frequency sequences and two of lower frequency. Each of the high-frequency sequences comprises a thick TSLA overlaying a thin TSLT, separated by a SIM surface. The exception is the first sequence on the top, corresponding to the upper two thirds of the Miranga Zone, where there is a reduction in the level of the lake and the record of a lowstand system tract (TSLB) composed of amalgamated channels, which erode the TSLA of the previous sequence. In terms of lower frequency, Sequence A on the top and Sequence B were identified. Sequence A is constituted predominantly by a TSLT, comprises the whole Santiago Zone, whose top is marked by the SIM (defined in the position of the Marker 11 regional log marker). Between the SIM and the LS, there is a TSLA that contains, in addition to a pelitic thick section, the base of the Miranga Zone. Above the LS is the TSLB of Sequence B, constituted by amalgamated channels with erosional base, followed by a TSLT and finally a TSLA. In summary, the interval studied configures a retrogradational pattern that respects the patterns observed by other authors at an interval of greater amplitude between the log marks of 15 to 7 (Pojuca Formation). This pattern is broken only in the base of Sequence A, where erosion relating to the reduction of the lake was interpreted, Disparity between the high-frequency stratigraphic sequences and the production zoning was also observed.