NMR Measurements in Carbonate Rocks: Problems and an Approach to a Solution

Carbonate rocks are well known for their complex petrophysical behavior where, in contrast to siliciclastic rocks, different parameters, including porosity and permeability, usually are not directly related. This behavior is the result of thorough reorganization of porosity during diagenesis, and it turns prediction of reservoir quality of carbonate rocks into a challenge. The study presented here deals with the problem of utilizing NMR techniques in prediction of petrophysical properties in carbonates.We employ a visual porosity classification as a priori knowledge for better interpreting NMR data for prediction purposes. This allows for choice of suitable T₂ cutoff values to differentiate movable from bound fluids adapted for the specific carbonate rock, thus resulting in better interpretation of NMR data. The approach of using a genetic pore type classification for adapting the conventional method for T₂ cutoff determination, which originally was developed for siliciclastic rocks, is promising. Similarly, for permeability determination on the basis of NMR measurements, the classification of carbonate rocks based on porosity types also shows potential. The approach implemented here has the promise to provide a basis of standardized interpretation of NMR data from carbonate rocks.Acknowledgment We are grateful to Baker Hughes INTEQ, Celle, for permission to publish the results of this work. This study was part of I.S.s Masters Thesis at Hannover University that was carried out in collaboration with Baker Hughes INTEQ, Celle, Germany. We appreciate comments of an anonymous referee and PAGEOPH editor Brian J. Mitchell.     

The ichnology of carbonate drifts

Carbonate drifts have so far not been as intensely investigated as their siliciclastic equivalents, especially from an ichnological perspective. The aim of this work is therefore to provide an overview of the different bioturbation styles in carbonate drifts for ichnologists and sedimentologists working in such deposits. Different types of carbonate drifts from the Maldives were studied to address this objective. The cores recovered during International Ocean Discovery Program Expedition 359 were examined to provide the sedimentological and ichnological data for a detailed analysis of the ichnology of carbonate drifts. The ichnological characteristics of the Maldives drifts are compared to other carbonate drifts in order to discuss similarities and differences, and thus provide an overview of the general characteristics of carbonate drift ichnology. These drifts are located in the Santaren Channel which lines Great Bahama Bank, along the Marion Plateau in Australia, in the Limassol and Larnaca basins in Cyprus and in the Danish Basin in Denmark. The common characteristics of bioturbation in carbonate drifts are: (i) the complete bioturbation of the sediment with bioturbation indexes between four and six; (ii) the occurrence of distinctive trace fossils limited to facies contacts or condensed intervals; (iii) a typical ichnoassemblage consisting of Thalassinoides, Scolicia, Planolites, Zoophycos, Chondrites, Phycosiphon and Palaeophycus; (iv) the contiguous occurrence of ichnogenera from different tiers, with only Zoophycos and Chondrites as deep tiers; and (v) distinct infills of the traces including particulate organic matter, pyrite, silica and celestine. In addition, the main ichnofacies of carbonate drifts is the Zoophycos ichnofacies. Ichnofabrics grade from coarse‐grained and completely bioturbated to ichnofabrics with present to rare trace fossils and preserved sedimentary structures. The type and intensity of the bioturbation is controlled by the amount of organic matter and the oxygenation at the sea floor that is determined by the action of bottom currents and the sea‐level fluctuations affecting the carbonate factory in carbonate platforms bordering the basins where the carbonate drifts form. The study of the bioturbation in core and outcrop provides palaeoenvironmental information about carbonate‐drift deposits that complement the classical sedimentological data.     

缝洞型碳酸盐岩气藏多类型储集层孔隙结构特征及储渗能力——以四川盆地高石梯-磨溪地区灯四段为例

为全面表征缝洞型碳酸盐岩气藏多类型储集层的孔隙结构特征及储渗能力,借助多种测试技术对四川盆地高石梯-磨溪地区灯四段储集层样品进行分析与研究。首先利用铸体薄片和扫描电镜技术定性刻画了储集层的岩性、物性、储集空间和喉道特征,然后根据高压压汞得到的毛管压力曲线对储集层进行分类,最后基于多尺度CT扫描定量表征了3类样品的二维、三维孔隙结构特征。结果表明：研究区储集空间既有受组构控制的粒间溶孔、粒内溶孔和晶间溶孔等,又有不受组构控制的溶洞、溶缝和构造缝;喉道以缩颈、片状和管束状为主;根据毛管压力曲线特征,储集层可划分为缝洞型、孔洞型和孔隙型;缝洞型大孔隙与溶洞发育,分布均匀且连通性好,喉道粗大且数量较多,微裂缝与溶洞串接呈串珠状分布,沟通了孤立的储集空间,具有最好的储渗能力;孔洞型多尺度孔隙与溶洞发育,储集能力强,喉道粗大但数量较少,连通性较差,各储集空间无法有效沟通,渗流能力受限;孔隙型细小孔隙发育且分布不均,大部分区域被岩石骨架占据,喉道数量少且连通性极差,储渗能力弱。     

Crystallization pathways in the Great Artesian Basin (Australia) spring mound carbonates: Implications for life signatures on Earth and beyond

Recent studies of continental carbonates revealed that carbonates with similar fabrics can be formed either by biotic, biologically-induced, biologically-influenced or purely abiotic processes, or a combination of all. The aim of this research is to advance knowledge on the formation of carbonates precipitated (or diagenetically altered) in extreme, continental environments by studying biotic versus abiotic mechanisms of crystallization, and to contribute to the astrobiology debate around terrestrial analogues of Martian extreme environments. Both fossil (upper Pleistocene to Holocene) and active carbonate spring mounds from the Great Artesian Basin (South Australia) have been investigated. These carbonates consist of low-Mg to high-Mg calcite tufa. Four facies have been described: (i) carbonate mudstone/wackestone; (ii) phytohermal framestone/boundstone; (iii) micrite boundstone; and (iv) coarsely crystalline boundstone. The presence of filaments encrusted by micrite, rich in organic compounds, including ultraviolet-protectants, in phytohermal framestone/boundstone and micrite boundstone is clear evidence of the existence of microbial mats at the time of deposition. In contrast, peloidal micrite, despite commonly being considered a microbial precipitate, is not directly associated with filaments in the Great Artesian Basin mounds. It has probably formed from nanocrystal aggregation on colloid particulate. Thus, where biofilms have been documented, it is likely that bacteria catalyzed the development of fabrics. It is less certain that microbes induced calcium carbonate precipitation elsewhere. Trace elements, including rare earth element distribution from laminated facies, highlight strongly evaporative settings (for example, high Li contents). Carbon dioxide degassing and evaporation are two of the main drivers for an increase in fluid alkalinity, resulting in precipitation of carbonates. Hence, although the growth of certain fabrics is fostered by the presence of microbial mats, the formation of carbonate crystals might be independent from it and mainly driven by extrinsic factors. More generally, biological processes may be responsible for fabric and facies development in micritic boundstone whilst micrite nucleation and growth are driven by abiotic factors. Non-classical crystallization pathways (aggregation and fusion of nanoparticles from nucleation clusters) may be more common than previously thought in spring carbonate and this should be carefully considered to avoid misinterpretation of certain fabrics as by-products of life. It is proposed here that the term ‘organic-compound catalyzed mineralization’ should be used for crystal growth in the presence of organic compounds when dealing with astrobiological problems. This term would account for the possibility of multiple crystallization pathways (including non-classical crystallization) that occurred directly from an aqueous solution without the direct influence of microbial mats.     

Fluid sources and stable isotope signatures in authigenic carbonates from the Northern Apennines,Italy

Authigenic carbonates are frequently associated with methane cold-seep systems, which extensively occur in various geologic settings worldwide. Of interest is the relation between the fluids involved in their formation and the isotopic signals recorded in the carbonate cements. Along the Northern Apennines foothills (Italy), hydrocarbons and connate waters still seeping nowadays are believed to be the primary sources for the formation of fossil authigenic carbonate found in Plio-Pleistocene marine sediments. Four selected outcrops of dolomitic authigenic carbonates were analysed to compare signature of seeping fluids with fractionation of stable carbon and oxygen isotopes recorded in the carbonate.Along the foothills, deep methane-rich fluids spontaneously rise to the surface through mud volcanoes or are exploited in wells drilled nearby to the fossil Plio-Pleistocene authigenic carbonates. The plumbing system providing fluids to present-day cold seeps was structurally achieved in Late Miocene and Plio-Pleistocene. δ¹³C values of methane, which vary from −51.9 to −43.0‰ VPDB, indicate that gas composition from the deep hydrocarbon reservoirs is relatively uniform along the foothills. On the contrary, δ¹³C in fossil authigenic carbonates strongly varies among different areas and also within the same outcrop.The different carbon sources that fed the investigated carbonates were identified and include: thermogenic methane from the deep Miocene reservoirs, ¹³C-enriched CO₂ derived from secondary methanogenesis and microbial methane from Pliocene successions buried in the Po Plain. The δ¹³C variability documented among samples from a single outcrop testifies that the authigenic carbonates might represent a record of varying biogeochemical processes in the hydrocarbon reservoirs. The sources of stable oxygen isotopes in authigenic carbonates are often ascribed to marine water. Oxygen isotopic fractionation in the dolomite cements indicates that marine pore water couldn't be the sole source of oxygen. δ¹⁸O values provide a preliminary evidence that connate waters had a role in the carbonates precipitation. The concomitant occurrence of active cold seepages and fossil record of former plumbing systems suggests that generation and migration of hydrocarbons are long-lasting and very effective processes along the Northern Apennines foothills.     

麦盖提斜坡奥陶系碳酸盐岩碳氧同位素特征及其意义

依据实测的塔里木盆地麦盖提斜坡玉北地区41个碳酸盐岩碳氧同位素数据,结合岩石学方法,研究了碳氧同位素的组成、演化及其地质意义。数据显示,δ~(13) C值主要分布在-2.6‰~0.7‰,均值为-1.0‰;δ~(18) O值分布在-9.4‰~-3.5‰,均值为-6.9‰。玉北地区古盐度为118.39~126.34,平均为121.94。奥陶系碳酸盐岩淡水改造作用明显。碳氧同位素的组成和演化不但可以指示沉积环境,而且还与生物生产率以及古海平面变化呈正相关性:δ~(13) C的低值对应于局限台地台内滩亚相沉积环境;δ~(13) C的高值对应于开阔台地滩间海、台内滩亚相沉积环境。碳氧同位素组成还对成岩环境有明显响应:鹰山组δ~(13) C与δ~(18) O均向高负值偏移,表明经历过强烈的表生岩溶作用;蓬莱坝组δ~(13) C低—中负值,δ~(18) O表现为高负值,在白云岩储层中可见鞍状白云石及燧石,主要为深埋藏成岩环境;良里塔格组同位素特征为δ~(18) O高负值,δ~(13) C低正值,并且在进入埋藏岩溶阶段之前还经历过风化壳岩溶作用。     

Seismic characterization of switching platform geometries and dominant carbonate producers (Miocene,Las Negras,Spain)

The primary goals of seismic interpretation and quantification are to understand and define reservoir architecture and the distribution of petrophysical properties. Since seismic interpretation is associated with major uncertainties, outcrop analogues are used to support and improve the resulting conceptual models. In this study, the Miocene carbonates of Cerro de la Molata (Las Negras, south‐east Spain) have been selected as an outcrop analogue. The heterogeneous carbonate rocks of the Cerro de la Molata Platform were formed by a variety of carbonate‐producing factories, resulting in various platform morphologies and a wide range of physical properties. Based on textural (thin sections) and petrophysical (porosity, density, carbonate content and acoustic properties) analyses of the sediments, eleven individual facies types were determined. The data were used to produce synthetic seismic profiles of the outcrop. The profiles demonstrate that the spatial distribution of the facies and the linked petrophysical properties are of key importance in the appearance of the synthetic seismic sections. They reveal that carbonate factory and facies‐specific reflection patterns are determined by porosity contrasts, diagenetic modifications and the input of non‐carbonate sediment. The reflectors of the seismograms created with high‐frequency wavelets are coherent with the spatial distribution of the predefined facies within the depositional sequences. The synthetic seismograms resulting from convolution with lower frequency wavelets do not show these details – the major reflectors coincide with: (i) the boundary between the volcanic basement and the overlying carbonates; (ii) the platform geometries related to changes in carbonate factories, thus sequence boundaries; and (iii) diagenetic zones. Changes in seismic response related to diagenesis, switching carbonate producers and linked platform geometries are important findings that need to be considered when interpreting seismic data sets.     

湖泊无机碳酸盐矿物氧同位素组成差值及其单矿物同位素组成的确定

湖积物中不同无机碳酸盐矿物常常混杂在一起,其氧同位素组成(δ18O)差异会影响碳酸盐δ18O环境信息提取的可靠性。不同矿物之间δ18O差值明显且幅度不一。20~25℃时生成的白云石比共生的方解石富集18O可能为0‰~9‰不等,亦或方解石比白云石可能更富集18O达0‰~12.3‰。常温状态,相同条件下形成的文石δ18O值较方解石高出0‰~1‰,或者方解石较文石δ18O值高出0‰~4.47‰。镁方解石中MgCO3的mol百分含量每增加1%,其δ18O值相对于纯方解石δ18O值增加0.06‰~0.17‰。在利用碳酸盐δ18O进行气候及环境研究时,不能根据某种差值进行校正,而应进行单矿物测试。由此,对不同无机碳酸盐矿物的分离及同位素测试、推算方法进行了归纳和述评。     

Origin of CO_2 in natural gas from the Triassic Feixianguan Formation of Northeast Sichuan Basin

The natural gas from the Triassic Feixianguan Formation of Northeast Sichuan Basin contains high H2S whereas relatively low CO2 concentrations and the CO2 display high δ13C values (ranging from -5.81‰ to 3.3‰ (PDB)). This seems to contradict the conventional wisdom that TSR should be a primary source of CO2 in natural gas from the Feixianguan Formation. In contrast, many authigenic calcite samples from these sites display very low δ13C values (ranging from -18.4‰ to -10.3‰ (PDB)). This suggests that the carbon from TSR source dominated the formation of calcite whereas the carbon from inorganic source came into CO2 in natural gas. In order to assess the origin of CO2 from these H2S-rich sites, we have calculated the relative contributions of organic and inorganic carbon sources to the CO2 and authigenic calcite. The organic carbon source possibly originated from TSR, whereas the inorganic one might be generated from marine carbonates dissolution. This calculation is based on the carbon isotopic compositions of CO2 and authigenic calcite as well as an isotopic mass balance. The results show that the contribution of organic carbon source to the CO2 is only 2%, whereas that to authigenic calcite is as high as 43% on average. Such results combined with thermodynamic evidence indicate that the isotopically light CO2 produced by TSR process may contribute to authigenic calcite precipitation during burial diagenesis. Distinguishable from Ordovician reservoir of Tarim Basin, Feixianguan reservoir of Northeast Sichuan Basin experienced rapid tectonic uplift due to Yanshanian movement after TSR occurred. Such tectonic event could induce temperature decrease and further promote carbonates dissolution. During these processes, secondary porosity has developed in Feixianguan carbonate reservoirs. Therefore, much attention should be paid to the structural highs in search of high quality carbonate reservoirs.     

Carboniferous foraminifers from the lower part of Paleo-Tethyan seamount-type carbonates in the Changning-Menglian Belt, western Yunnan, Southwest China

The Changning-Menglian Belt in West Yunnan, Southwest China is well-known as a closed remnant of the Paleo-Tethys Ocean in East Asia (Wu et al., 1995; Liu et al., 1996). It is delineated to the east with the Lincang Massif by the Changning-Shuangjiang Fault and to the west with the Baoshan Block by the Kejie-Nandinghe Fault, and is generally subdivided into three zones: east, central, and west zones. In the central zone, various kinds of oceanic rocks such as harzburgite, cumulate websterite, gabbro, both mid-oceanic ridge basalt and oceanic island basalt, Devonian-Triassic radiolarian chert, and Carbonifer-ous-Permian massive and huge carbonates with basaltic effusives as their pedestal are exposed (Liu et al., 1991, 1996; Wu et al., 1995; Ueno et al., 2003). These Central zone rocks are now interpreted to have been emplaced as nappes structurally overlying the East and West zones, which are considered as consisting mainly of passive margin sediments of the Baoshan Block (Wu, 1991; Ueno et al., 2003).