The 'Turkana Grits' : Potential Hydrocarbon Reservoirs of the Northern and Central Kenya Basins
The 'Turkana Grits' : Potential Hydrocarbon Reservoirs of the Northern and Central Kenya Basins
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“图尔卡纳砂粒”：肯尼亚盆地北部和中部的潜在油气藏
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发表时间：
2015
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通讯作者：
G. Muia
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作者：
G. Muia
世界上超过三分之二的巨型油田分布在两个主要的构造体系中：大陆被动边缘和大陆裂谷。裂谷中碳氢化合物的优先形成归因于高品位湖相烃源岩单元与中至高品位储集岩的近端并置--这是断裂作用和由此产生的可容纳空间中的沉积作用的结果，在许多情况下，这可能会局部改变当时的气候条件。在这样的盆地之一，肯尼亚裂谷的洛基查尔盆地，已经发现了超过6亿桶的可采石油。该盆地的主要储集层单元是Lokone砂岩，它属于一个更大的砂岩家族，称为图尔卡纳砂岩，即夹在变质基底和中中新世火山岩之间的硬砂岩。罗肯砂岩作为储集层单元的油气倾向性推动了图尔卡纳砂岩作为潜在油气藏的进一步研究。通过详细测井，研究了以往尚未从年代地层学和沉积学的角度对金威尔组、卡梅戈组和洛里乌砂岩三个沉积建造进行了详细的研究。采集了170多个样品，确定了碎屑组分和自生组分，确定了不同露头的主要胶结带，并通过岩相分析确定了沉积环境。火山岩和侵入岩样品也被表征，并用于39Ar-40Ar测年。确定了金威尔组、远端河道、冲积扇和漫滩沉积环境的三种叠加沉积环境。成岩研究表明，胶结物由底部的赤铁矿为主向中部带内的方解石转变，并向地层顶部返回赤铁矿。这些胶结作用发生在低温条件下(<；80°C)的早成岩和相对晚成岩作用，在显著的机械压实作用下。金威尔组的最小沉积年龄也被确定为大约18 Ma(中新世早期-布隆迪)。卡米戈组发育河流-泛滥平原沉积环境，主要由赤铁矿胶结。方解石水泥只出现在最下面的5米处。与卡米戈组顶部沉积物互层的薄熔岩流给出了大多数沉积物的最小沉积年龄约为20 Ma。洛里乌砂岩主要由河流河道沉积组成。胶结物主要为方解石、赤铁矿和高岭石粘土。横贯岩墙的最小沉积年龄约为18.5 Ma。对图尔卡纳砂砾岩的最终储层分析表明，虽然压实作用和胶结作用是减少孔隙度的主要因素，但图尔卡纳砂砾岩通常是较差至中等好的储集单元。已证实，Lokone Sanstone的地下孔隙率在10-20%之间，渗透率高达3达里尔(非洲石油公司，2011年)。在岩石学分析方面，金威尔组被列为第二好的储集层潜力，在其研究的地层中，某些区段的孔隙率高达20%。卡米戈组也有很好的潜力，但排名不高，因为火山岩物质的巨大成分更容易发生成岩蚀变。洛里乌砂岩没有良好的孔隙度，因此在图尔卡纳砂岩中排名第五。
Over two thirds of the world’s giant oilfields are found in two principle tectonic regimes; continental passive margins and continental rifts. The preferential formation of hydrocarbons in rifts is attributed to the proximal juxtaposition of high grade, lacustrine source rock units with medium to high grade reservoir rocks - a consequence of both faulting and sedimentation in the resulting accommodation space, which in many cases may locally modify the prevailing climatic conditions. In one of such basins, the Lokichar Basin in the Kenyan Rift, over 600 million barrels of recoverable oil have been discovered. The principle reservoir unit in this basin is the Lokone Sandstone that belongs to a larger family of sandstones called the ‘Turkana Grits’, arkosic sandstones that are sandwiched between metamorphic basement and mid-Miocene volcanics. The hydrocarbon proclivity of the Lokone Sandstones as reservoir units motivated further study of the ‘Turkana Grits’, as potential hydrocarbon reservoirs. In this work, three sedimentary formations, i.e. Kimwarer Formation, Kamego Formation and Loriu Sandstones, which have not been previously fully characterized from chronostratigraphic and sedimentological point of views were studied through detailed logging. Over 170 samples were collected to determine, detrital and authigenic components, the main cementation zones in the different outcrops, and, from lithofacies analysis, the depositional environments. Volcanic and intrusive samples were also characterized and used for 39Ar-40Ar dating. Three superposed depositional environments were determined for the Kimwarer Formation, a distal fluvial channel, an alluvial fan and a floodplain depositional environment. The diagenetic study shows cements change from dominant hematite at the base to calcite within the middle zones and back to hematite towards the top of the Formation. These cementation episodes occur during early and relatively late diagenesis in low temperature conditions (<80 °C), under significant mechanical compaction. A minimum deposition age at ca. 18 Ma (Early Miocene – Burdigalian) has also been set for the Kimwarer Formation. The Kamego Formation evolves from fluvial to floodplain depositional environments and is dominantly cemented by hematite. Calcite cement is only noted in the lowermost 5m. A thin lava flow interbedded with the topmost sediments of the Kamego Formation gave a minimum deposition age of ca. 20 Ma for most of the sediments. The Loriu Sandstone is composed predominantly of fluvial channel deposits. The main cements are calcite, hematite and kaolinite clays. A cross-cutting dyke suggests a minimum deposition age of ca. 18.5Ma. A final reservoir analysis of the Turkana Grits shows that while compaction and cementation are dominant agents of porosity reduction, the Turkana Grits are generally poor to moderately good reservoir units. The Lokone Sanstone has been proven to have sub-surface porosities ranging between 10 - 20% and permeabilities as high as 3 darcies (Africa Oil Corporation, 2011). For petrographic analyses, the Kimwarer Formation has been ranked as having the second best reservoir potential with porosities as high as 20% in some sections of its studied stratigraphy. The Kamego Formation also has good potential but is not as highly ranked owing to the huge component of volcanic material that have a greater propensity to diagenetic alteration. No good porosities were noted for the Loriu Sandstone and hence this formation has been ranked 5th amongst the Turkana Grits