pre-Tertiary Carbonates Research Articles

A geological overview of the limestone members of the Woyla Group of Sumatra, Indonesia

Mesozoic limestone units of the Woyla group were identified in many places across the northern part of Sumatra, Indonesia. Even though these sedimentary rocks may play an important role as an element of the potential Pre-Tertiary hydrocarbon play of Sumatra, their characteristics are still not well understood. This study tries to fill this research gap and aims to better understand the characteristics of the limestone members of the Woyla group. There are three objectives of this study: (1) to characterise structural features, and deformation of the Woyla Group; (2) to provide sedimentary characteristics of the limestone members of the Woyla Group; and (3) to understand the main influences on the development of the limestone members of the Woyla Group. An integrated geological analyses, including structural scanline analysis, petrographic analysis, and acid digestion analysis, was conducted to achieve the objectives of this study.
 Findings from this research show that the limestone members of the Woyla group were strongly deformed, and structural features such as bedded strata, faults, folds, and joints were identified within these rocks. The limestone units of the Woyla group consist of at least six microfacies. These are wackestone, packstone, wackestone-packstone, packstone-rudstone, fossiliferous sandstone, and fossiliferous shale. Depositional processes, sea level fluctuations, tectonisms, and climatic variations are interpreted as the main factors influencing the development and evolution of these limestone units. It is expected that the results of this study could advance our understanding of the Pre-Tertiary carbonate rocks in general, and the Woyla group of Sumatra in particular.

Dongfang 1-1 gas field in the mud diapir belt of the Yinggehai Basin, South China Sea

The Dongfang 1-1 (DF1-1) mud diapir field is the largest gas field discovered in the Yinggehai Basin up to now, with a proven gas reserve of 99.68×10 9 m 3. The gas pay zones are within the Pliocene fine sandstones and siltstones of the littoral and sand bar facies, with normal pressure regimes. The gases include variable amounts of CO 2 and N 2 in addition to hydrocarbon gases. The hydrocarbon gases (with the exception of biogenic methane) and nitrogen originated from the marine source rocks within the overpressured Lower to Middle Miocene strata, while CO 2 is most likely the product of thermal decomposition of the Tertiary calcareous mudstones and pre-Tertiary carbonates through relatively long-distance vertical migration. The field has received at least four phases of natural gas charging occurring relatively late, most likely during the post-Pliocene Himalayan orogeny. Diapiric faults act as the main conduit for upward movement of fluids released from the deep high-pressure system, with abnormally high pressures providing the significant driving force for gas migration. Because of the episodic nature of the deep high-pressured fluid release and the diapir fault activity, this shallow diapir gas field is characterized by multiple gas sources and episodic fluid accumulation.

Geochemistry and episodic accumulation of natural gases from the Ledong gas field in the Yinggehai Basin, offshore South China Sea

The Ledong gas field, consisting of three gas pools in a shale diapir structure zone, is the largest gas discovery in the Yinggehai Basin. The gases produced from the Pliocene and Quaternary marine sandstone reservoirs show a considerable variation in chemical composition, with 5.4–88% CH 4, 0–93% CO 2, and 1–23.7% N 2. The CO 2-enriched gases often display heavier methane δ 13C values than those with low CO 2 contents. The δ 15N values of the gases range from −8 to −2‰, and the N 2 content correlates negatively with the CO 2 content. The high geothermal gradient associated with a relatively great burial depth in this area has led to the generation of hydrocarbon and nitrogen gases from the Lower–Middle Miocene source rocks and the formation of abundant CO 2 from the Tertiary calcareous-shales and pre-Tertiary carbonates. The compositional heterogeneities and stable carbon isotope data of the produced gases indicate that the formation of the LD221 gas field is attributed to three phases of gas migration: initially biogenic gas, followed by thermogenic hydrocarbon gas, and then CO 2-rich gas. The filling processes occurred within a short period approximately from 1.2 to 0.1 Ma based on the results of the kinetics modeling. Geophysical and geochemical data show that the diapiric faults that cut through Miocene sediments act as the main pathways for upward gas migration from the deep overpressured system into the shallow normal pressure reservoirs, and that the deep overpressure is the main driving force for vertical and lateral migration of the gases. This gas migration pattern implies that the transitional pressure zone around the shale diapir structures was on the pathway of upward migrating gases, and is also a favorable place for gas accumulation. The proposed multiple sources and multiple phases of gas migration and accumulation model for the Ledong gas field potentially provide useful information for the future exploration efforts in this area.

Multiphase natural gas migration and accumulation and its relationship to diapir structures in the DF1-1 gas field, South China Sea

The Dongfang1-1(DF1-1) gas field is the largest commercial gas field yet discovered in the Yinggehai Basin, South China Sea. The gas reservoirs in the field are normally pressured Pliocene shallow marine facies fine-grained sandstones, and the main source rocks are highly over-pressured Miocene mudstones and shales. The natural gases in the reservoirs consist mainly of hydrocarbon gas, CO 2 and N 2. Geochemical data for the natural gases show that the hydrocarbon-rich gases (except for a small amount of biogas) were derived from these source rocks distributed within over-pressured zones, but gases rich in CO 2 mainly originate from the thermal decomposition of Miocene calcareous shales and pre-Tertiary carbonates at higher temperature. Fluid inclusion data for the reservoirs were used together with the regional geology, distribution of different types of reservoirs and the evolution of diapir structures to determine the episodes of gas migration and accumulation in the DF1-1 gas field. We believe that the field was formed by four episodes of gas migration and accumulation. Fluid paleopressure simulation software, designed and developed by CNOOC China Limited, was used to simulate the paleopressure history of the strata at the top of the present-day high pressure zone (a depth of 2800 m) in the DF1-1 gas field. It predicts that the first episode occurred before about 2 Ma, the second and the third episodes between 2 and 0.5 Ma, and that the CO 2-rich gases migrated into the reservoirs after 0.5 Ma. The formation and evolution of diapir structures in the Yinggehai Basin had an important influence on the migration and accumulation of natural gases in the DF1-1 gas field. Rapid deposition, gas generation in the source rocks and thermal expansion of pore-fluids caused over-pressuring in the Miocene shales in the center of the depression. This over-pressuring together with active faulting, formed diapir structures during 5–0.5 Ma. The faults and fractures related to the diapir structures provided a pathway for gas vertical migration from the deep source rocks into the DF1-1 reservoirs, and controlled the geological distribution of different types of natural gases. On this basis, a model for gas migration and accumulation is suggested for the DF1-1 gas field.

Detection of groundwater conduits in limestones with gravity surveys: data from the area of the Chicxulub Impact crater, Yucatan Peninsula, Mexico.

Small negative gravity anomalies are found in gravity data from along the northwestern shoreline of the Yucatan Peninsula. These anomalies are shown to be due to elongate, shallow anomalous porosity zones in the Tertiary carbonates. These zones are caused primarily by groundwater solution and are presently active conduits for groundwater flow. The association of these small gravity anomalies with known topographic and structural features of the area, which partially overlies the Chicxulub Impact crater, indicates their development was influenced by structures, faults and/or fractures, within the Tertiary and pre-Tertiary carbonates.

Petrographic composition and depositional environments of the Pliocene Velenje lignite seam (Slovenia)

A Pliocene lignite seam up to 160 m thick occurs in the Velenje depression in northern Slovenia. It forms part of a non-marine transgressive sequence. Petrographic data, together with ash and sulphur content of samples from two boreholes (one located in the central, and the other in the marginal part of the seam) have been used to study vertical and lateral coal facies variations. In the central part, the lignite seam overlies fluvial sediments. Facies critical macerals and petrographic indices suggest that the lower (ash rich) part of the seam (unit I) accumulated in a wet forest swamp in an alluvial plain setting. Later this environment was replaced by a dry (telmatic) forest swamp (unit IIa). According to our interpretation, a decrease in tree density in units IIb and IIc reflects a change to a bush moor. The transition from unit II to unit III is characterized by a rapid water table rise, which resulted in the establishment of a wet (limnotelmatic) forest swamp. Within unit III, the tree density decreases and the degree of gelification increases towards the top of the seam suggesting a transition to a fen type environment. Another rapid water table rise ended peat formation in the eutrophic, topogenous mire, and established open lacustrine conditions. A calcium rich environment resulted in high pH values and is responsible for high sulphur lignites (up to 4% on a dry basis). In the upper part of the seam, the pH values were controlled by the amount of the inflowing lacustrine water. Only thin, ash-rich topotelmites accumulated along the northern margin of the mire. There, the seam originated in a wet forest swamp and was separated by limnic and fluvial sediments from other organic rich horizons. Due to its closer vicinity to the pre-Tertiary carbonates, the depositional environment was probably even more alkaline. Therefore, the lignite shows a higher degree of gelification and is even more sulphur rich (up to 5.5% on a dry basis).