Mafic

Key Takeaways

• The compositional spectrum from mafic to felsic igneous rocks reflects the fundamental differentiation of magmas by partial melting and fractional crystallization of the Earth's mantle and crust: mantle-derived primary magmas are basaltic (mafic) in composition, reflecting the olivine, pyroxene, and spinel mineralogy of the peridotite mantle; as these primary basaltic magmas rise and partially crystallize in the lower crust, the early-crystallizing mafic minerals (olivine and pyroxene, which incorporate Mg and Fe preferentially) settle out, leaving a residual magma progressively enriched in silica, alkali elements, and incompatible trace elements that eventually produces the more silicic (intermediate to felsic) magma compositions represented by andesite, dacite, and rhyolite; the continuous spectrum from mafic (basalt, 45 to 52 percent SiO2) through intermediate (andesite, 52 to 63 percent SiO2) to felsic (rhyolite and granite, above 63 percent SiO2) reflects the degree of fractionation from the primary mafic melt, with ultramafic rocks (below 45 percent SiO2, such as peridotite, dunite, and komatiite) representing the most primitive, least fractionated compositions closest to the mantle source.
• Mafic intrusions (sills and dykes) into sedimentary basins affect petroleum systems in multiple ways, both as heat sources for source rock maturation and as barriers or conduits to fluid flow depending on their geometry and diagenetic history: igneous sills intruded into organic-rich shales or coals can rapidly heat the surrounding sediment (the contact thermal aureole extending several sill thicknesses above and below the intrusion) to temperatures sufficient for oil or gas generation, effectively accelerating maturation over geological timescales; the Neuquen Basin of Argentina, the Karoo Basin of South Africa, and the Voring Margin of offshore Norway all have examples where Cretaceous or Jurassic mafic sills intruded Jurassic or Triassic source rocks and generated hydrocarbons that subsequently migrated into structural traps; however, the emplacement of mafic sills at reservoir level can also completely destroy reservoir quality by thermal metamorphism of the host rock and precipitation of diagenetic minerals (chlorite, calcite, zeolites) in the pore system of the surrounding sedimentary rock, creating a low-permeability aureole around the intrusion that baffles fluid flow.
• Mafic basement weathering creates saprolite and laterite profiles that in some geological settings represent potential hydrocarbon reservoir rocks (fractured basement reservoirs), particularly in basement highs overlain by source-rock-bearing sedimentary sequences where hydrocarbons generated in the source rock can migrate downward into the fractured and weathered basement: the pre-Cambrian crystalline basement reservoirs of Vietnam (producing from granite and gneiss at Bach Ho and other fields), the Paleozoic igneous and metamorphic basement of the Huangliu Formation in China, and the fractured basement reservoirs of Libya (Sirte Basin) all include mafic and intermediate igneous basement rocks with secondary fracture and vug porosity that hosts commercial petroleum accumulations; the porosity in these basement reservoirs is entirely secondary (fracture and weathering-derived), with the primary crystalline rock having essentially zero porosity and permeability; the seismic expression of basement reservoir targets differs from conventional sedimentary reservoirs (no bright amplitude anomalies correlated with fluid content, irregular top surface reflecting paleotopography of the erosional unconformity, and chaotic internal seismic reflectivity) and requires specialized interpretation techniques.
• Petrographic identification of mafic rocks and minerals in well cuttings, core samples, and thin sections uses the combination of mineral color, crystal form, cleavage, and specific gravity: mafic minerals (olivine, pyroxene, amphibole, and biotite) are characteristically dark green, black, or greenish-black in hand specimen due to their high iron content, with specific gravities of 3.2 to 4.4 grams per cubic centimeter that make them sink rapidly in heavy liquid mineral separation tests; the diagnostic mafic minerals in basalt include augite (monoclinic pyroxene, eight-sided cross-section, two cleavages at nearly 90 degrees), plagioclase (striated twinning in hand specimen, biaxial negative optical sign), and olivine (high relief, irregular fracture, no cleavage in thin section), with their proportions and grain size indicating the crystallization rate (coarse-grained gabbro crystallized slowly at depth; fine-grained basalt crystallized rapidly at the surface); in drilling operations, encountering mafic igneous rock in cuttings when the well program predicted sandstone or limestone indicates either a basement penetration (reaching the igneous basement beneath the sedimentary section) or an intrusion (a sill or dyke cutting through the sedimentary section), with significant implications for reservoir depth prediction and remaining well objectives.
• Mafic ocean floor basalts form the oceanic crust that underlies all of the world's ocean basins, produced continuously at mid-ocean spreading ridges where mantle-derived mafic magmas upwell and cool to form the pillow lavas, sheeted dykes, and layered gabbros that constitute the ophiolite sequence: the average composition of oceanic crust is very close to that of mid-ocean ridge basalt (MORB), with approximately 50 percent SiO2, 10 percent FeO, 8 percent MgO, and 12 percent CaO; as oceanic crust is subducted beneath continental margins at convergent plate boundaries, the mafic composition of the subducting slab influences the composition of the arc magmas generated by partial melting above the subducting plate, with fluids released from the subducting slab (water from hydrated mafic minerals, CO2 from carbonates) lowering the melting temperature of the mantle wedge and generating the calc-alkaline magmas (intermediate to felsic composition) that are characteristic of island arc and continental margin volcanic systems; the recognition that subducting mafic oceanic crust drives arc volcanism is the foundation of plate tectonic theory's explanation for the global distribution of volcanic arcs and their associated metallic ore deposits.