加蓬G4-188区块钻井技术实践认识

加蓬G4-188区块是一个新的海外勘探区块,在钻井过程中遇到了井壁垮塌、气侵、井斜控制困难等诸多难题,影响了钻井施工的速度。针对这些难题,实施了解决G4-188区块钻井施工难点的各项技术措施,为G4-188区块后续钻井施工提供了有益的技术支持。     

加蓬海岸盆地南、北次盆成藏特征对比

加蓬海岸盆地属于被动大陆边缘盆地。盆地沉积盖层主要由白垩系、古近系和新近系组成,沉积地层组合具有明显的三分性,包括早白垩世盐下层系、早白垩世晚期阿普特期盐膏层和晚白垩世至新近纪盐上层系。根据目前的油气发现,北加蓬主要富集盐上油气资源,南加蓬主要富集盐下油气资源。从构造沉积背景、油气地质条件、油气发现规模、油气平面分布、油气勘探潜力5方面对南、北次盆进行对比,分析南、北次盆的油气聚集差异,结合勘探现状分析其勘探潜力,并指出南加蓬次盆勘探难点为圈闭识别,北加蓬次盆的勘探难点是砂体预测。     

Late-Holocene equatorial environments inferred from deposition processes,carbon isotopes of organic matter,and pollen in three shallow lakes of Gabon,west-central Africa

Seven vibro-cores were collected from three shallow lakes of the Gabon (Kamalété, Nguène, Maridor) along a 300-km west–east transect close to the Equator. These lakes are located in very distinct landscapes: coastal forest-savanna mosaic, rain forest and savanna with colonising forest, respectively. Core chronologies were established by radiocarbon dating. Study of these lacustrine archives (textural variables, clay minerals, organic matter components, δ¹³C, pollen) allowed comparison of late Holocene environmental changes recorded at each site and with results from other studies. Lake Kamalété indicates minor climatic deterioration (increased drying and greater seasonality) between 1,410 and 500 cal. years BP, which is also recognised in southern Cameroon and east-central Africa. Lake Nguène was surrounded by dense moist forest throughout the last 4,110 years, but shows significant deterioration from ~2,800 cal. years BP, a phenomenon seen at nearby sites. Lake Maridor shows a decline of forest initiated a little after 3,800 cal. years BP, which indicates timing that is distinct from the two other sites. This was probably a response to local conditions (i.e. outlet damming). Although the three lakes display generally parallel climatic trends perhaps linked to SST oscillations, there is not perfect coherence between these three sites. Differences among the three basins may be attributable to local factors like groundwater hydrology and slope instabilities of such shallow lake systems in this equatorial region.     

西非加蓬海岸盆地盐岩特征及其石油地质意义

加蓬海岸盆地主要分为南加蓬次盆和北加蓬次盆,是典型的西非被动大陆边缘含盐盆地,盐岩的分布具有一定规律性和差异性,并且对盆地内油气成藏具有重要的控制作用。分析了加蓬海岸盆地的构造演化与沉积充填特征、基本石油地质条件、盐岩的分布特征及其对盆地烃源岩特征、储层分布、圈闭特征、封盖条件、油气成藏,以及油气藏储量等各方面的影响与控制作用,并指出,由于加蓬海岸盆地的绝大多数油气成藏都与盐岩密不可分,因此深化盆地内盐岩展布特征的研究,对进一步评价北加蓬次盆的盐下层系及南次盆登泰尔地堑的勘探潜力具有重要意义。     

Lowland rainforest response to hydrological changes during the last 1500 years in Gabon, Western Equatorial Africa

Pollen and δ¹³C_TOM data obtained from two contrasting lake sequences (Lakes Kamalété and Nguène), located 200 km apart in the lowland rainforest of Gabon, provide complementary local and regional 1500-yr records of high resolution (15–30 yr) vegetation change. A combination of aquatic, semi-aquatic and terrestrial pollen showed in both records that the tropical rainforest increased during periods of high rainfall and decreased during drought intervals. The strong fluctuations of water balance at decadal scale during the “Medieval Warm Period” ( 1100–800 cal yr BP) coincided with a noticeable increase in shade-intolerant taxa, indicating recurring rainforest canopy disturbance. The δ¹³C_TOM signal showed high-amplitude variations in both records, which positively correlates with the rainforest dynamics and local vegetation changes. The similar trends in both the pollen and the δ¹³C_TOM signals between these sites demonstrate the regional broadly synchronous timing of shifting hydrological conditions. The largely positive co-variation between strong fluctuations of hydrological conditions and changes in rainforest structure and composition indicate that regional climatic change is probably the driving force for major rainforest dynamics in Gabon. Any significant anthropogenic impact on vegetation has not been clearly identified, and this issue still needs to be resolved independently by obtaining detailed archeological records across the interval 1400–800 BP, which currently seem to be extremely rare or not easily available.     

Percolation of diagenetic fluids in the Archaean basement of the Franceville basin

The Palaeoproterozoic Franceville basin, Gabon, is mainly known for its high-grade uranium deposits, which are the only ones known to act as natural nuclear fission reactors. Previous work in the Kiéné region investigated the nature of the fluids responsible for these natural nuclear reactors. The present work focuses on the top of the Archaean granitic basement, specifically, to identify and date the successive alteration events that affected this basement just below the unconformity separating it from the Palaeoproterozoic basin. Core from four drill holes crosscutting the basin–basement unconformity have been studied. Dating is based on U–Pb isotopic analyses performed on monazite. The origin of fluids is discussed from the study of fluid inclusion planes (FIP) in quartz from basement granitoids. From the deepest part of the drill holes to the unconformable boundary with the basin, propylitic alteration assemblages are progressively replaced by illite and locally by a phengite + Fe chlorite ± Fe oxide assemblage. Illitic alteration is particularly strong along the sediment–granitoid contact and is associated with quartz dissolution. It was followed by calcite and anhydrite precipitation as fracture fillings. U–Pb isotopic dating outlines three successive events: a 3.0–2.9-Ga primary magmatic event, a 2.6-Ga propylitic alteration and a late 1.9-Ga diagenetic event. Fluid inclusion microthermometry suggests the circulation of three types of fluids: (1) a Na–Ca-rich diagenetic brine, (2) a moderately saline (diagenetic + meteoric) fluid, and (3) a low-salinity fluid of probable meteoric origin. These fluids are similar to those previously identified within the overlying sedimentary rocks of the Franceville basin. Overall, the data collected in this study show that the Proterozoic–Archaean unconformity has operated as a major flow corridor for fluids circulation, around 1.9 Ga.     

Mapping of aboveground biomass in Gabon

The aim of this paper is to map the aboveground biomass (AGB) in Gabon. First, a random forest (RF) model that relates reference AGB values to remote sensing (RS)-derived variables (mainly radar and optical images) was built, and the significant predictive variables were determined. Second, the built RF model was applied to the significant RS-derived variables to predict AGB across Gabon. The results showed that the overall RMSE (Root Mean Square Error) on the RS-derived AGB map with a spatial resolution of 50 m was 63.3 t/ha (R² = 0.53).To improve the accuracy of the RS-derived AGB map, the integration of LiDAR data provided by the Geoscience Laser Altimeter System (GLAS) onboard the Ice Cloud and Land Elevation Satellite (ICESat) was investigated. First, an RF model that relates reference AGB values to GLAS-derived metrics and a DEM (Digital Elevation Model) was built. Second, the calibrated RF model was applied to obtain a spatially distributed estimation of AGB (GLAS footprints geolocation) covering forested areas in Gabon, with a density of 0.13 footprints/km². Third, the semivariogram of residuals (RS-derived AGB map – GLAS-derived AGB “surrogate AGB”) was computed. Later, a regression kriging interpolation was performed by taking into account the spatial structure of residuals to provide a continuous residual map. Finally, the RS-derived AGB map and the residual map were summed, and a final AGB map was obtained. The results showed that the integration of GLAS surrogate AGB data slightly improves the accuracy of the RS-derived AGB map only for AGB values lower than 100 t/ha (bias and RMSE reduced by 13.9 and 10 t/ha, respectively).     

加蓬盆地盐岩特征及其对盐下油气勘探的影响

加蓬盆地是一个典型的被动大陆边缘含盐盆地,以下白垩统盐岩为界分为盐上、盐下两个勘探领域。近年来随着巴西深水盐下陆续获得多个世界级油气田发现,以加蓬、宽扎为代表的西非盐下也逐渐成为世界油气勘探的热点。如何落实盐下圈闭是制约盐下油气勘探的首要难题。从盐岩岩性组合、岩石物理特征入手,分析了加蓬盐岩特征及其对盐下地震成像及构造落实的影响。同时,针对加蓬深水这一热点区域勘探程度低、烃源岩、储层等基本石油地质条件不清的特点,利用盆地模拟手段,分析了加蓬深水区盐下烃源岩的热成熟度及盐岩对盐下烃源岩热演化的影响;结合新钻井资料,分析了盐岩对盐下储层物性的影响,指出加蓬深水区盐下储层具有较好物性;分析了盐岩平面分布特征,从盖层角度指出了盐下油气勘探最有利的区带。对其他含盐盆地盐下油气勘探具有一定借鉴意义。     

SHRIMP in situ isotopic analyses of REE,Pb and U in micro-minerals bearing fission products in the Oklo and Bangombé natural reactors: A review of a natural analogue study for the migration of fission products

In the Precambrian, parts of the Oklo, Okélobondo and Bangombé uranium deposits of the Republic of Gabon, central Africa, functioned as natural fission reactors. Many elements in the Oklo and Bangombé uranium deposits show variations in isotopic composition caused by a combination of nuclear fission, neutron capture and radioactive decay. Isotopic studies provide useful information to understand the behavior of radionuclides in geological media. In our recent work, in situ REE, Pb and U isotopic analyses of individual tiny minerals in and around reactor zones have been performed using a SHRIMP (Sensitive High Resolution Ion Microprobe). The isotopic results of the SHRIMP analyses on micro-minerals found in and around the Oklo and Bangombé natural reactors are reviewed in this paper. The data suggest the selective uptake behavior of (1) Ra into illite, and (2) Pu into apatite, (3) the formation process of secondary minerals bearing fissiogenic REE and depleted U, (4) evidence of nuggets (?-particles) bearing fissiogenic platinum group elements (PGE), and (5) from the U–Pb systematics of highly altered zircons, the redistribution of U and Pb.