Julian C. Baker

Inferred syngenetic textural evolution in Holocene cryptic reefal microbialites, Heron Reef, Great Barrier Reef, Australia

Cryptic microbialites in the Heron Reef framework occur as crusts of fingerlike microcolumns or branching dendrolites, rarely more than 1 cm long. Microstructure of the most recently growing microbialite surfaces consists of coalesced, 3 µm scalenohedra that are indistinguishable from previously described Mg-calcite “abiotic” cement. The transformation from submicrometer, anhedral crystallites to >3 µm scalenohedra is inferred to have occurred only during active microbialite accretion beneath a biofilm. This syngenetic change from primary, biologically induced microstructures to microstructures that are indistinguishable from abiotic cement has important implications for the recognition and interpretation of early marine microcrystalline carbonates and cements.

Diagenesis and Petrophysics of the Early Permian Moogooloo Sandstone, Southern Carnarvon Basin, Western Australia

The fluvio-deltaic Moogooloo Sandstone is an attractive petroleum exploration target because it is favorably positioned with respect to source and seal units. To assess and predict reservoir quality distribution in the Moogooloo Sandstone and laterally equivalent Keogh Formation, a petrological and petrophysical calibration data set was established by acquiring extensive core petrology, porosity, permeability, and thermal maturity data.

Permian Evolution of Sandstone Composition in a Complex Back-Arc Extensional to Foreland Basin: The Bowen Basin, Eastern Australia

ABSTRACT The Bowen Basin is a Permo-Triassic, back-arc extensional to foreland basin that developed landward of an intermittently active continental volcanic arc associated with the eastern Australian convergent plate margin. The basin has a complex, polyphase tectonic history that began with limited back-arc crustal extension daring the Early Permian. This created a series of north-trending grabens and half grabens which, in the west, accommodated quartz-rich sediment derived locally from surrounding uplifted continental basement. In the east, coeval calc-alkaline, volcanolithic-rich, and volcaniclastic sediment was derived from the active volcanic arc. This early extensional episode was followed by a phase of passive thermal subsidence accompanied by episodic compression during the late Early Permian to early Late Permian, with little contemporaneous volcanism. In the west, quartzose sediment was shed from stable, polymictic, continental basement immediately to the west and south of the basin, whereas volcanolithic-rich sediment that entered the eastern side of the basin during this time was presumably derived from the inactive, and possibly partly submerged, volcanic arc. During the late Late Permian, flexural loading and increased compression occurred along the eastern margin of the Bowen Basin, and renewed volcanism took place in the arc system to the east. Reactivation of this arc led to westward and southward spread of volcanolithic-rich sediment over the entire basin. Accordingly, areas in the west that were earlier receiving quartzose, craton-derived sediment from the west and south were overwhelmed by volcanolithic-rich, arc-derived sediment from the east and north. This transition from quartz-rich, craton-derived sediments to volcanolithic-rich, arc-derived sediments is consistent with the interpreted back-arc extensional to foreland basin origin for the Bowen Basin.

ESEM study of authigenic chlorite acid sensitivity in sandstone reservoirs

The effect of HCl on authigenic chlorite in three different sandstones has been examined uisng an Environmental Scanning Electron Microscope (ESEM), together with conventional analytical techniques. The ESEM enabled chlorites to be directly observed in situ at high magnifications during HCl treatment, and was particularly effective in allowing the same chlorite areas to be closely compared before and after acid treatment. Chlorites were reacted with 1M to 10M HCl at temperatures up to 80°C and for periods up to five months. After all treatments, chlorites show extensive leaching of iron, magnesium and aluminum, and their crystalline structure is destroyed. However, despite these major compositional and structural changes, chlorites show little or no visible evidence of acid attack, with precise morphological detail of individual plates preserved in all samples following acid treatments. Chlorite dissolution, sensu stricto, did not occur as a result of acidization of the host sandstones. Acid-treated chlorides are likely to exits in a structurally weakened state that may make them susceptible to physical disintegration during fluid flow. Accordingly, fines migration may be a significant engineering problem associated with the acidization of chlorite-bearing sandstones.

ESEM study of illite/smectite freshwater sensitivity in sandstone reservoirs

The water sensitivity of authigenic smectite- and illite-rich illite/smectites in sandstone reservoirs has been investigated using an Environmental Scanning Electron Microscope (ESEM). The ESEM enabled the illite/smectites to be directly observed in situ at high magnification during freshwater immersion, and was also particularly effective in allowing the same selected illite/smectite areas to be closely compared before and after freshwater treatments. The tendency of authigenic smectite-rich illite/smectite to swell on contact with fresh water varies greatly. Smectite-rich illite/smectite may osmotically swell to many times its original volume to form a gel which greatly reduces porosity and permeability, or may undergo only a subtle morphological change which has little or no adverse effect on reservoir quality. Authigenic illite-rich illite/smectite in sandstones does not swell when immersed in fresh water. Even after prolonged soaking in fresh water, illite-rich illite/smectite particles retain their original morphology. Accordingly, illite-rich illite/smectite in sandstones is unlikely to cause formation damage if exposed to freshwater-based fluids.

Diagenesis and Reservoir Quality of Paleocene Sandstones in the Kupe South Field, Taranaki Basin, New Zealand

Paleocene reservoir sandstones in the Kupe South field, Taranaki basin, contain a diagenetic mineral assemblage that records major shifts in pore-water composition during the burial history of the basin. Early calcite formed at shallow burial largely from meteoric depositional pore waters, whereas later chlorite/smectite records the downward passage of marine pore waters into the sandstones from overlying, marine mudrocks prior to significant sandstone compaction during the late Miocene. Late calcite and ferroan carbonates may record the presence of connate meteoric water expelled upward from nonmarine sedimentary rocks of the underlying Cretaceous sequence, whereas later kaolinite and secondary porosity formation are related to localized meteoric influx resulting from la e Miocene to early Pliocene uplift and erosion of the reservoir section. Hydrocarbon entrapment occurred during further Pliocene to Holocene sediment accumulation. Labile-grain alteration has been less severe in the lower part of the hydrocarbon-bearing section (lower sands) than in the upper part (upper sands), with the result that the lower sands contain mainly chlorite/smectite, and the upper sands contain mainly ferroan carbonates and kaolinite formed by extensive alteration of labile grains and earlier-formed chlorite/smectite. Reservoir quality in the lower sands is controlled mostly by grain size and the presence of chlorite/smectite, but in the upper sands, the presence of kaolinite is the single most important cause of poor reservoir quality.

Occurrence and palaeohydrological significance of authigenic kaolinite in the Aldebaran sandstone, Denison Trough, Queensland, Australia

Thin section, XRD, SEM, and isotopic techniques have been used to study authigenic kaolinite occurring in reservoir sandstones of the Lower Permian Aldebaran Sandstone. Where the unit is no longer an active aquifer, kaolinite is an intermediate-stage phase, and is highly depleted in deuterium (δDSMOW = −115 to −99‰) and 18O (δ18OSMOW = +7.8 to +8.9‰), indicating that precipitation must have been from meteoric water. Deep penetration of this water is linked to Late Triassic deformation and uplift of the Denison Trough sequence, an event which led to exposure of the Aldebaran Sandstone by the Early Jurassic prior to its re-burial beneath Jurassic and Cretaceous sedimentary rocks. The same water was probably involved in the creation of secondary porosity in the interval.Where the Aldebaran Sandstone is presently undergoing meteoric flushing, kaolinite is relatively enriched in deuterium (δDSMOW = −104 to −93‰) and 18O (δ18OSMOW = +11.7 to +14.6‰), reflecting precipitation largely from post-Mesozoic meteoric water which was isotopically heavier than the Mesozoic water involved in intermediate-stage kaolinite precipitation. This temporal shift in meteoric water isotopic composition is related to the northward drift of the Australian continent to lower latitudes since the Mesozoic Era.

Oxygen-isotope evidence for upward, cross-formational porewater flow in a sedimentary basin near maximum burial

Abstract Authigenic ankerite in the gas-bearing mid-Permian Aldebaran Sandstone (Denison Trough, Queensland, Australia) has an anomalously light oxygen-isotopic composition ( δ 18 O SMOW range: +7.6 to +14.4‰ ) and exhibits a trend of 18O-enrichment from the base to the top of the unit. Textural relationships, together with burial and thermal modelling, indicate that this carbonate precipitated at temperatures of about 100 to 140°C, when the sequence approached maximum burial during the Late Triassic. This implies that ankerite precipitated from porewater very depleted in 18O with respect to marine water ( δ 18 O SMOW = −9 to −5‰ ). The formation of this deep, relatively high-temperature ankerite is difficult to reconcile with downward percolation of meteoric water at that time since the basin was then undergoing its first burial/compactional cycle. We interpret the ankerite to have precipitated from porewater expelled upward from the earliest Permian Reids Dome beds. This thick unit, consisting mainly of high-latitude continental sandstones, mudrocks and coals, was initially saturated with very 18O-depleted meteoric water ( δ 18 O SMOW ≈ −17‰ ) partly derived from melted snow and ice, and is likely to have undergone overpressuring during rapid burial (at rates up to 1 km/Ma). Tectonically induced expulsion of “connate meteoric” porewater out of the Reids Dome beds took place as the sequence approached maximum burial prior to Late Triassic basin uplift. This water was flushed upward through the overlying units, retaining a (modified) meteoric isotopic signature, which was recorded by the precipitating ankerite. Computer modelling of heat transport, isotopic mass balance and water mixing quantitatively shows that this interpretation is viable, lending support to the suggested mechanism of upward, cross-formational porewater flow deep in a sedimentary basin.

Freshwater sensitivity of corrensite and chlorite/smectite in hydrocarbon reservoirs — an ESEM study

Abstract An Environmental Scanning Electron Microscope (ESEM) has been used to investigate the freshwater sensitivity of secondary corrensite (regularly interstratified chlorite/smectite) and chlorite-rich chlorite/smectite in order to determine whether hydrocarbon reservoirs hosting these clays should be regarded as freshwater sensitive. ESEM experiments involved an examination and close comparison of selected clay areas in three samples at high magnification before, during and after prolonged freshwater treatments. Corrensite and chlorine/smectite in the samples did not visibly swell when immersed in fresh water. After soaking in fresh water for up to three months, these clays retained their original morphology and associated porosity. Hence, the presence of corrensite or chlorite/smectite in a hydrocarbon reservoir need not indicate that the reservoir is freshwater sensitive.

Freshwater sensitivity of glauconitic hydrocarbon reservoirs

Abstract Freshwater sensitivity tests were conducted on four glauconite/smectite mixed-layer clays (G/S) with 10–20% smectite content in order to determine whether G/S with low expandability has the potential to swell on contact with fresh water. Tests showed that three of the G/S samples underwent no visible swelling when immersed for up to 4 days in fresh water. In contrast, one sample was freshwater sensitive because, in addition to non-swelling G/S with low expandability, it contained a small component of highly smectitic G/S that rapidly swelled on contact with fresh water. These results indicate that the presence of G/S with low expandability in a hydrocarbon reservoir should not render the reservoir freshwater sensitive, but that individual reservoirs can contain a compositionally-heterogeneous population of G/S grains that may include a highly freshwater-sensitive, smectitic component.