Origins of CO2 in Permian carbonate reservoir rocks (Zechstein, Ca2) of the NW-German Basin (Lower Saxony)
Origins of CO2 in Permian carbonate reservoir rocks (Zechstein, Ca2) of the NW-German Basin (Lower Saxony)
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DOI：
10.1016/j.chemgeo.2005.09.014
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发表时间：
2006-04
期刊：
影响因子：
3.9
通讯作者：
M. Fischer;R. Botz;M. Schmidt;K. Rockenbauch;D. Garbe‐Schönberg;J. Glodny;P. Gerling;R. Littke-R.-Littk
中科院分区：
文献类型：
--
作者：
M. Fischer;R. Botz;M. Schmidt;K. Rockenbauch;D. Garbe‐Schönberg;J. Glodny;P. Gerling;R. Littke-R.-Littk
CO2区（德国西北部下萨克森盆地）二叠纪碳酸盐岩储集岩（Stassfurt-Carbonate）的局部特征是CO2含量高，这给勘探带来了严重问题。为了获得关于CO2主要来源和CO2运移和聚集过程的信息，对Stasslavian碳酸盐岩（Ca 2）中的自生沉淀物进行了矿物学、地球化学和稳定同位素（13 C，18 O）分析。这些岩石含有早期成岩作用（例如，在软沉积物中形成）碳酸盐矿物（方解石，白云石）和硬石膏在结核和小裂缝。此外，大型（宽达2厘米）碳酸盐胶结裂缝形成固体Ca 2岩石。成岩矿物含量的半定量估计显示，所调查的岩芯之间的变化。在12个Ca 2岩心中，有9个来自位于现代富甲烷储层区域的威尔斯井，并发育有明显的裂缝系统。相比之下，其他三个岩心来自以富CO2储层为特征的威尔斯井，并且缺乏类似厚度的裂缝。构造裂缝与Ca 2储集岩含气量之间的假定关系意味着Ca 2储集岩在约102 °C至150 °C（假定δ 18 O孔隙水变化范围为0‰至+4‰ SMOW）时发生CO2脱气。稀土元素分布表明，至少有三种类型的裂缝和裂缝填充碳酸盐。两个碳酸盐生成演化成岩孔隙沃茨，而一种类型反映热液影响下的碳酸盐形成。Rb/Sr和U/Pb同位素年龄分析表明，这些胶结物形成于晚白垩世下萨克森盆地反转晚期或反转之后。自生碳酸盐矿物的稳定碳同位素组成反映了成岩过程中至少有两个碳源端元可供利用。而许多早期形成的碳酸盐岩与二叠纪富含13 C的碳源有关，这些碳酸盐岩在早期到晚期成岩作用期间沉淀或重结晶和/或由局部构造事件引起，这些碳酸盐岩从有机降解过程中富集了12 C。自生碳酸盐的稳定氧同位素值可能仍然反映了二叠纪海水中矿物形成的低温（如对“非”重结晶碳酸盐的假设）或来自海水孔隙沃茨的后期高温（高达150 °C）沉淀，这些水可能受到局部富18 O流体的影响。然而，大多数的碳酸盐样品的研究是重结晶（观察到的结核和裂缝碳酸盐的晶体放大），因此，这些碳酸盐至少部分再平衡（original 18 O-含量减少）与周围的孔隙沃茨在高温下。与自生碳酸盐类似，至少有两个来源可以解释Ca 2储集岩中现代CO2的浓度/碳同位素关系。有机源CO2端元源（δ 13 C接近−20‰）的浓度相对较低，而同位素值接近0‰的端元源CO2浓度较高。虽然后者的来源是未知的，这种“重”CO2源最有可能归因于碳酸盐分解过程。
Permian carbonate reservoir rocks (Stassfurt-Carbonate) of the CO2-province (Lower Saxony Basin, NW Germany) are locally characterised by high amounts of CO2leading to serious problems on exploration. In order to receive information on the major CO2sources and CO2migration and accumulation processes mineralogical, geochemical and stable isotope analyses (13C,18O) were carried out on authigenic precipitations within the Stassfurt carbonate rocks (Ca2). These rocks contain early diagenetic (e.g., formed in soft sediment) carbonate minerals (calcite, dolomite) and anhydrite in concretions and small cracks. In addition, large (up to 2 cm wide) carbonate-cemented fractures formed in solid Ca2 rocks. Semi-quantitative estimates of the diagenetic mineral content showed variations between the investigated rock cores. Nine out of 12 Ca2 rock cores are derived from wells situated in the area of modern CH4-rich reservoirs and have distinct fracture systems developed. In contrast, the other three rock cores are from wells characterised by CO2-rich reservoirs and lack fractures of similar thickness. The assumed relationship between tectonically induced fractures and the gas content of the Ca2 reservoir rocks implies CO2degassing of the Ca2 rocks at approximately 102 °C to ∼150 °C (assumed δ18Opore watervariation from 0‰ to +4‰ SMOW). REE distribution indicates at least three types of fracture- and crack-filling carbonates. Two carbonate generations formed from evolving diagenetic pore waters whereas one type reflects carbonate formation under hydrothermal influence. The ages of these cementations derived from Rb/Sr and U/Pb isotope analyses indicate their formation during late stage of/or after the inversion of the Lower Saxony Basin in Late Cretaceous times. The stable carbon isotope composition of the authigenic carbonate minerals reflects at least two carbon source endmembers available during diagenesis. Whereas many early-formed carbonates are related to the Permian13C-rich carbon source, those carbonates which precipitated or recrystallised during early to late stage diagenesis and/or were induced by local tectonic events incorporated12C-enriched CO2from organic degradation processes. The stable oxygen isotope values of authigenic carbonates probably still reflect low temperatures of mineral formation from Permian sea water (as it is assumed for “non” recrystallised carbonates) or late-stage high-temperature (up to ∼150 °C) precipitation from sea water-derived pore waters which may have been locally influenced by18O-enriched fluids. However, most of the carbonate samples investigated are recrystallised (crystal enlargement of concretionary and fracture carbonate is observed) and hence, these carbonates are at least partly re-equilibrated (original18O-content reduced) with surrounding pore waters at elevated temperature. Similar to the authigenic carbonate at least two sources may explain the concentration/carbon isotope relationship of modern CO2present in the Ca2 reservoir rocks. An organic-derived CO2endmember source (δ13C near −20‰) is present in relatively low concentrations whereas large CO2concentrations derived from an endmember source with an isotope value near 0‰. Although the latter source is unknown such “heavy” CO2sources are most likely attributed to carbonate decomposition processes.