Figure 5. Photomicrographs show main lithologies and porosity types of the Montoya Aleman pay zone. A: Coarse-/very coarse-grained, skeletal grainstone, the main chert-free lithofacies of the Aleman Formation, with diverse skeletons including echinoderm (E), bryozoan (B), brachiopod (Br), and trilobite (Tr), some echinoderm having been collophane-replaced (Co), synaxial calcite cemented, fine dolomite occurring along muddy stylolite (St), thin-section from G. Lyda 1H, 15468 feet. B: Fine-grained, skeletal grainstone, one of the two most abundant host rocks of the pay chert, composed of well sorted skeletal fragments of echinoderm (E), ostracod (O), bryozoan (B), and trilobite and equant and synaxial calcite cements, with minor intraparticle pores (blue), thin-section from G. Lyda 1H, 15474 feet. C: Fine-grained, echinoderm (E) dominated, skeletal packstone, the other abundant host rock of the pay chert, well-sorted, slightly dolomitized (D), with minor collophane-replaced grains (Co), Stained thin-section from Rape 14-1H, 13626 feet. D: Reduced interparticle porosity in chert, remnant textures showing initially a skeletal grainstone with abundant echinoderms and ostracods, stained thin-section from Rape 14-1H, 13627 feet. E: Moldic porosity (blue) after dissolved spicules in spiculitic limy-chert, stained thin-section from Rape 14-1H, 13616 feet. F: Small pore system (bright blue) and micro-porosity in chert, resulted from continued precipitation of silica in the early interparticle spaces and remained intraparticle pores of silicified echinoderms, pore throats less than 0.05 mm, stained thin-section from Rape 14-1H, 13616 feet.