Title: Genesis and reservoir properties of hydrothermal Dolomites (HTD), Ramales Platform (Northern Spain)
Other Titles: Genese en reservoir kenmerken van hydrothermale dolomieten (HTD), Ramales Platform (Noord-Spanje)
Authors: Dewit, Julie
Issue Date: 22-Nov-2012
Abstract: The gradual exhaustion of classic hydrocarbon reservoirs and increasing global energy demand, lead to the exploration of new, more complex oil and gas plays. Hydrothermal dolomites (HTDs), which form by the interaction of a limestone host rock with high temperature dolomitising brines, represent a newly discovered type of reservoir rock. Ironically, the first commercial hydrocarbon discovery in a carbonate reservoir (i.e. the Lima-Indiana field in the Trenton Formation) was in a HTD. These HTDs are not only of great interest for their economic aspects, but are also a matter of much debate in fundamental research. At this time, some questions regarding the genesis of HTDs remain unanswered and the parameters controlling their characteristics and their distribution are virtually unknown. Over the last decades many studies focussed on the genesis of HTD, but only a limited number investigated their (reservoir) characteristics. Moreover, the latter are generally based on a limited set of cores or borehole logs, which hampers the investigation of the distribution of the HTDs’ characteristics. In this study, both the diagenesis and characteristics of different HTD bodies are addressed. This requires the combination of a classic diagenetic study including petrography, geochemistry and fluid inclusion microthermometry on the one hand and a qualitative and/or quantitative characterisation of the HTD bodies by digital outcrop model analysis, systematic sampling, standard petrophysical measurements and (geo)-statistics on the other hand. The area selected for this study is located in northern Spain in the Ramales Platform, which developed in the Basque-Cantabrian Basin (BCB) during the Aptian and Albian. This carbonate platform hosts several relatively small fault-controlled HTD bodies in the platform edge and large stratabound HTD bodies in inner-platform deposits. Both types of HTD bodies are associated with deep seated sinistral strike-slip faults.The fault-controlled HTD bodies exposed in the platform edge are characterised by a systematic spacing and mainly develop along their feeder faults. The largest fault-controlled HTD body, defined as the Pozalagua body, occurs along the Pozalagua Fault. Along this fault a horizontal displacement of 500 m was inferred and more joint sets developed compared to the rest of the platform edge. The combination of these observations suggests that the Pozalagua body developed along the most active fault, i.e. accommodating most of the sinistral strike-slip deformation. In addition, this Pozalagua body developed more extensively in inner-platform deposits. There, the dolomite-limestone contact is controlled by three joint sets (N30W, N40E and N75E) which could be related to the development of Riedel shears in a sinistral strike-slip regime. Part of the Pozalagua body crops out north of the Ramales platform edge. It is defined as the Pico del Carlista body and is selected for the quantitative study of its (reservoir) characteristics. Based on the macroscopic texture, three types of HTD aredistinguished, i.e. matrix, coarse crystalline and zebra dolomite. The spatial distribution of these types is characterised by clusters of the same type and the preferential occurrence of matrix dolomite and the host rock at neighbouring sites. Two directions of anisotropy can be observed in the spatial distribution of the three HTD types in the Pico del Carlista body, i.e. N30-35W and N60E, which correspond to the strike of the Pozalagua Fault and the platform edge. The latter two are interpreted as the main fluid conduits for the Pico del Carlista body. Three zones dominated by a specific type of HTDs can be defined in this body. 1) Along and close to the fluid conduits coarse crystalline dolomite occurs. This is interpreted as the result of the recrystallisation of earlier matrix and zebra dolomite phases. 2) In the central part of the body, zebra dolomites dominate. Since zebra dolomites preferentially develop along faults or in fault zones the occurrence of the zebra dolomite in the central part of the HTD body is related to the Pozalagua Fault zone. 3) At the dolomite-limestone contact matrix dolomite is the main HTD type. Despite the difference in texture and pore types observed in the matrix, coarse crystalline and zebra dolomite, based on plug analysis no significant difference in porosity and permeability was inferred for the three types. In general, the porosity of the HTD ranges between 1.8 and 12.1% and the permeability between 0.01 and 17.4 mD. No relation between porosity and permeability of the HTD (types) exists. These findings are based on an extensive dataset consisting of HTD types and poro-perm measurements of the HTDs sampled in the Pico del Carlista body. Comparison of the (geo)-statistical data obtained from the entire dataset with data obtained from a limited number of sections sampled, shows that studying one or two well-chosen sections would have yielded similar results. In the stratabound body of Matienzo, the HTDs’ characteristics are analysed in a qualitative way. The geometry of the HTD body is controlled by joints, bedding planes and the presence of massive limestone units and faults. No relation is observed between the precursor limestone facies and the occurrence of HTD, massive limestone units however form impermeable barriers for dolomitising fluids. Matrix, coarse crystalline and zebra dolomite can also be discerned in this HTD body. The distribution of these types in three sub-horizontal zones is interpreted as the result of fluid flow, degree of oversaturation relative to dolomite, water-rock interaction and the presence of faults. In the lower part of the HTD body, matrix dolomites are dominant and interpreted as the result of the reaction of a highly oversaturated dolomitising fluid with the host rock. As a consequence of the high degree of oversaturation, dolomite mineralisation starts on many nucleation points resulting in a fine crystalline dolomite. As the dolomitising fluid flows to the upper part of the body, the degree of oversaturation with respect to dolomite decreases. Dolomitewill nucleate on fewer points and not all the dissolved CaCO3 is replaced by CaMg(CO3)2. This explains the occurrence of a porous coarse crystalline dolomite in the upper part of the HTD body. The presence of zebra dolomite in the middle part of the HTD body is attributed to the injection of dolomitising fluids along tributary faults. The HTDs of the Matienzo body are characterised by similar porosity (1.8 - 13.7%) and permeability (0.01 - 4.4 mD) values as in the Pico del Carlista body, which are obtained from standard petrographic analyses on small plugs. The upper 80 m of the Matienzo HTD body, composed of porous coarse crystalline dolomite, represents a potentially interesting rock from a reservoir point of view. In the Pozalagua body, a saw-cut quarry exposes multiple diagenetic phases and allows the reconstruction of a detailed paragenetic sequence. In the rest of the study area the outcrops do not generally allow observation of all the diagenetic phases seen in the Pozalagua Quarry, but a consistent paragenetic sequence can be inferred for the entire study area. The diagenetic phases observed in the Ramales Platform are attributed to the eogenetic, mesogenetic or telogenetic realm depending on their relation with different sets of stylolites. Early diagenetic cements, which developed prior to burial stylolites, occlude most of the primary pores and no synsedimentary/early diagenetic dolomite is observed in the host limestone. Four diagenetic phases cross cut the first set of burial stylolites. These burial diagenetic phases consist of two calcite and two dolomite phases. The first phase is a coarse crystalline calcite (CCC). It differs from the other burial diagenetic phases present in the study area by its geographic distribution, i.e. it has only been observed in the Ramales Platform edge, and its composition, i.e. it is the only phase that precipitated from a H2O-NaCl-CaCl2 fluid. The parent fluid has a moderate salinity (with an average of 15.9 eq. wt. % NaCl) and a radiogenic Sr isotope signature. It is suggested that the parent fluid originated from dewatering of basinal sediments. This CCC is cross cut by sinistral shear veins cemented by dolomite. Though multiple sub-phases of dolomite can be observed in the Pozalagua Quarry, these can be grouped into a ferroan and a non-ferroan dolomite phase. Both major phases have similar cathode luminescence, stable and Sr isotope signatures, salinity and Th within each HTD body. The ferroan dolomites have however higher Fe and Mn concentrations and are associated with MVT ores. The depleted delta18O values, radiogenic Sr isotope signature, high salinity (18 eq. wt. % NaCl on average at Ranero) and high Th (140 – 160°C on average in the different bodies) of the dolomitising fluid combined with Pb isotope and crush-leach data reported in the literature indicates that the dolomitising fluid is a residual brine originating from the Triassic Keuper deposits of the BCB, which possibly interacted with basement rocks. The variation in Th for the different HTD bodies is attributed to a different relation between the HTD and their respective feeder fault systems. The HTDs are cross cut by a second calcite phase, i.e. a blocky calcite that occurs in sigmoidal veins and veins containing angular clasts of the host rock. Late calcite phases are reported over and over again in association with HTD. The parent fluid of this calcite is less saline (8.6 – 14.7 eq. wt. % NaCl) and has a lower Th (51 – 77°C) than the dolomitising fluid, which does not exclude a genetic link between both. The blocky calcite is the last burial diagenetic phase and is cross cut by burial and tectonic stylolites. A relative timing for the burial diagenetic phases was inferred based on literature of the BCB and observations made during this study. The association of the HTD with sinistral strike-slip faults leads to the conclusion that the HTD precipitated in a transtensional regime, which prevailed during the late Cretaceous. Additional indications for a late Cretaceous and more precisely late Albian age for the dolomites is given by the presence of hydrothermal vents along faults in the BCB, the cross-cutting relation of the CCC with Supra-Urgonian sandstones at Jorrios, the emplacement of volcanics along major faults of the BCB and the onset of metamorphism by the convection of hot fluids in the Pyrenees. Two late diagenetic phases are attributed to the telogenetic realm of the inverted BCB, i.e. the honey coloured calcite and associated dedolomite. The zoned cathode luminescence, low Sr concentration, the meteoric calcite line formed by the stable isotope signatures and the inferred delta18O VSMOW signatures of the parent fluid, support a meteoric origin of the honey calcite and by consequence of the associated dedolomite. The meteoric alteration of the dolomite and its host rock probably started during the Miocene. The results of this study can be used for the prediction of the occurrence of HTD bodies in basins that experienced a similar geodynamic evolution as the BCB. If fault-controlled and/or stratabound HTD bodies are encountered a comparable distribution of the HTD bodies and (reservoir) characteristics within the HTD bodies can be assumed.
ISBN: 978-90-8649-574-0
Publication status: published
KU Leuven publication type: TH
Appears in Collections:Division of Geology

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